The word for "angels" here is aggelos (ἄγγελος), and it means agents or messengers. (See [this post]). It could refer to spirit-agents, but there is nothing inherent in this passage that says it has to. It could also refer to human-agents.
Young's Literal Translation translates verse 14 like this:
saying to the sixth messenger who had the trumpet, 'Loose the four messengers who are bound at the great river Euphrates;'
The tense of the greek word translated "bound" is Perfect Passive Accusative Plural... or "ones having been bound".
This verse only says these agents were released from their bond at the River Euphrates. This verse says nothing about where they were bound.
Verse 15 says they were prepared for a particular hour and day and month and year - for a particular moment.
Also, in the very next chapter, the angel tells John that he has to prophesy again about something. What is it?
And the voice that I heard out of the heaven is again speaking with me, and saying, ‘Go, take the little scroll that is open in the hand of the messenger who hath been standing upon the sea, and upon the land.’
and I went away unto the messenger, saying to him, ‘Give me the little scroll;’ and he saith to me, ‘Take, and eat it up, and it shall make thy belly bitter, but in thy mouth it shall be sweet – as honey.’
And I took the little scroll out of the hand of the messenger, and did eat it up, and it was in my mouth as honey – sweet, and when I did eat it – my belly was made bitter;
and he saith to me, It behoveth thee again to prophesy about peoples, and nations, and tongues, and kings – many’.
Which means it is very likely this passage is referring to kings.
Here are some ancient texts that shed some light on this as well.
Read History of Alexander the Great of Macedonia 5.1.17 - 5.1.30:
Moving on to Babylon, Alexander was met by Mazaeus, who had taken refuge in the city after the battle. He came as a suppliant with his grown-up children to surrender himself and the city. Alexander was pleased at his coming, for besieging so well-fortified a city would have been an arduous task and, besides, since he was an eminent man and a good soldier who had also won distinction in the recent battle, Mazaeus’ example was likely to induce the others to surrender. Accordingly Alexander gave him and his children a courteous welcome.
Nevertheless, he put himself at the head of his column, which he formed into a square, and ordered his men to advance into the city as if they were going into battle.
A large number of the Babylonians had taken up a position on the walls, eager to have a view of their new king, but most went out to meet him, including the man in charge of the citadel and royal treasury, Bagophanes. Not to be outdone by Mazaeus in paying his respects to Alexander, Bagophanes had carpeted the whole road with flowers and garlands and set up at intervals on both sides silver altars heaped not just with frankincense but with all manner of perfumes.
Following him were his gifts - herds of cattle and horses, and lions, too, and leopards, carried along in cages.
Next came the Magians chanting a song in their native fashion, and behind them were the Chaldaeans, then the Babylonians, represented not only by priests but also by musicians equipped with their national instrument. (The role of the latter was to sing the praises of the Persian kings, that of the Chaldaeans to reveal astronomical movements and regular seasonal changes.)
At the rear came the Babylonian cavalry, their equipment and that of the horses suggesting extravagance rather than majesty.
Surrounded by an armed guard, the king instructed the townspeople to follow at the rear of his infantry; then he entered the city on a chariot and went into the palace. The next day he made an inspection of Darius’ furniture and all his treasure,
but it was the city itself, with its beauty and antiquity, that commanded the attention not only of the king, but of all the Macedonians. And with justification. Founded by Semiramis (not, as most have believed, Belus, whose palace is still to be seen there),
its wall is constructed of small baked bricks and is cemented together with bitumen. The wall is ten meters wide and it is said that two chariots meeting on it can safely pass each other.
Its height is twenty-five meters and its towers stand three meters higher again. The circumference of the whole work is 365 stades, each stade, according to the traditional account, being completed in a single day. The buildings of the city are not contiguous to the walls but are about thirty meter’s width from them,
and even the city area is not completely built up - the inhabited sector covers only 275 hectares - nor do the buildings form a continuous mass, presumably because scattering them in different locations seemed safer. The rest of the land is sown and cultivated so that, in the event of attack from outside, the besieged could be supplied with produce from the soil of the city itself.
The Euphrates passes through the city, its flow confined by great embankments. Large as these structures are, behind all of them are huge pits sunk deep in the ground to take water of the river when in spate, for when its level has exceeded the top of the embankment, the flood would sweep away city buildings if there were no drain shafts and cisterns to siphon it off.
These are constructed of baked brick, the entire work cemented with bitumen.
The two parts of the city are connected by a stone bridge over the river, and this is also reckoned among the wonders of the East. For the Euphrates carries along with it a thick layer of mud and, even after digging this out to a great depth to lay the foundations, one can hardly find a solid base for a supporting structure.
Moreover, there is a continuous build-up of sand which gathers around the piles supporting the bridge, impeding the flow of water, and this constriction makes the river smash against the bridge with greater violence than if it had an unimpeded passage.
Also read Library of History of Diodorus Siculus Book 17, Chapters 112, 116-118 for the messengers being loosed:
After the conclusion of his war with the Cossaeans, Alexander set his army in motion and marched towards Babylon in easy stages, interrupting the march frequently and resting the army.
While he was still three hundred furlongs from the city, the scholars called Chaldaeans, who have gained a great reputation in astrology and are accustomed to predict future events by a method based on age-long observations, chose from their number the eldest and most experienced. By the configuration of the stars they had learned of the coming death of the king in Babylon, and they instructed their representatives to report to the king the danger which threatened. They told their envoys also to urge upon the king that he must under no circumstances make his entry into the city;
that he could escape the danger if he re-erected the tomb of Belus which had been demolished by the Persians, but he must abandon his intended route and pass the city by.
The leader of the Chaldaean envoys, whose name was Belephantes, was not bold enough to address the king directly but secured a private audience with Nearchus, one of Alexander’s Friends, and told him everything in detail, requesting him to make it known to the king.
When Alexander, accordingly, learned from Nearchus about the Chaldaeans’ prophecy, he was alarmed and more and more disturbed, the more he reflected upon the ability and high reputation of these people. After some hesitation, he sent most of his Friends into Babylon, but altered his own route so as to avoid the city and set up his headquarters in a camp at a distance of two hundred furlongs.
This act caused general astonishment and many of the Greeks came to see him, notably among the philosophers Anaxarchus.
When they discovered the reason for his action, they plied him with arguments drawn from philosophy and changed him to the degree that he came to despise all prophetic arts, and especially that which was held in high regard by the Chaldaeans. It was as if the king had been wounded in his soul and then healed by the words of the philosophers, so that he now entered Babylon with his army.
As on the previous occasion, the population received the troops hospitably, and all turned their attention to relaxation and pleasure, since everything necessary was available in profusion.
These were the events of this year.
... just when it seemed that he was at the peak of his power and good fortune, Fate cut off the time allowed him by nature to remain alive. Straightway heaven also began to foretell his death, and many strange portents and signs occurred.
Once when the king was being rubbed with oil and the royal robe and diadem were lying on a chair, one of the natives who was kept in bonds was spontaneously freed from his fetters, escaped his guards’ notice, and passed through the doors of the palace with no one hindering.
He went to the royal chair, put on the royal dress and bound his head with the diadem, then seated himself upon the chair and remained quiet. As soon as the king learned of this, he was terrified at the odd event, but walked to the chair and without showing his agitation asked the man quietly who he was and what he meant by doing this.
When he made no reply whatsoever, Alexander referred the portent to the seers for interpretation and put the man to death in accordance with their judgement, hoping that the trouble which was forecast by his act might light upon the man’s own head. He picked up the clothing and sacrificed to the gods who avert evil, but continued to be seriously troubled. He recalled the prediction of the Chaldaeans and was angry with philosophers who had persuaded him to enter Babylon. He was impressed anew with the skill of the Chaldaeans and their insight, and generally railed at those who used specious reasoning to argue away the power of Fate.
A little while later heaven sent him a second portent about his kingship. He had conceived the desire to see the great swamp of Babylonia and set sail with his friends in a number of skiffs. For some days his boat became separated from the others and he was lost and alone, fearing that he might never get out alive.
As his craft was proceeding through a narrow channel where the reeds grew thickly and overhung the water, his diadem was caught and lifted from his head by one of them and then dropped into the swamp. One of the oarsmen swam after it and, wishing to return it safely, placed it on his head and so swam back to the boat. After three days and nights of wandering, Alexander found his way to safety just as he had again put on his diadem when this seemed beyond hope. Again he turned to the soothsayers for the meaning of all this.
They bade him sacrifice to the gods on a grand scale and with all speed, but he was then called away by Medius, the Thessalian, one of his Friends, to take part in a comus. There he drank much unmixed wine in commemoration of the death of Heracles, and finally, filling a huge beaker, downed it at a gulp.
Instantly he shrieked aloud as if smitten by a violent blow and was conducted by his Friends, who led him by the hand back to his apartments. His chamberlains put him to bed and attended him closely, but the pain increased and the physicians were summoned. No one was able to do anything helpful and Alexander continued in great discomfort and acute suffering. When he, at length, despaired of life, he took off his ring and handed it to Perdiccas. His Friends asked: “To whom do you leave the kingdom?” and he replied: “To the strongest.” He added, and these were his last words, that all of his leading Friends would stage a vast contest in honour of his funeral. This was how he died after a reign of twelve years and seven months. He accomplished greater deeds than any, not only of the kings who had lived before him but also of those who were to come later down to our time.
Since some historians disagree about the death of Alexander, and state that this occurred in consequence of a draught of poison, it seems necessary for us to mention their account also.
They say that Antipater, who had been left by Alexander as viceroy in Europe, was at variance with the king’s mother Olympias. At first he did not take her seriously because Alexander did not heed her complaints against him, but later, as their enmity kept growing and the king showed an anxiety to gratify his mother in everything out of piety, Antipater gave many indications of his disaffection. This was bad enough, but the murder of Parmenion and Philotas struck terror into Antipater as into all of Alexander’s Friends, so by the hand of his own son, who was the king’s wine-pourer, he administered poison to the king.
After Alexander’s death, Antipater held the supreme authority in Europe and then his son Casander took over the kingdom, so that many historians did not dare write about the drug. Casander, however, is plainly disclosed by his own actions as a bitter enemy to Alexander’s policies. He murdered Olympias and threw out her body without burial, and with great enthusiasm restored Thebes, which had been destroyed by Alexander.
After the king’s death Sisyngambris, Dareius’s mother, mourned his passing and her own bereavement, and coming to the limit of her life she refrained from food and died on the fifth day, abandoning life painfully but not ingloriously.
Having reached the death of Alexander as we proposed to do at the beginning of the book, we shall try to narrate the actions of the Successors in the books which follow.
This period of history is also covered in the book of Daniel.
Read Daniel 8:1-8:
'In the third year of the reign of Belshazzar the king, a vision hath appeared unto me -- I Daniel -- after that which had appeared unto me at the beginning.
And I see in a vision, and it cometh to pass, in my seeing, and I am in Shushan the palace that is in Elam the province, and I see in a vision, and I have been by the stream Ulai.
And I lift up mine eyes, and look, and lo, a certain ram is standing before the stream, and it hath two horns, and the two horns are high; and the one is higher than the other, and the high one is coming up last.
I have seen the ram pushing westward, and northward, and southward, and no living creatures do stand before it, and there is none delivering out of its hand, and it hath done according to its pleasure, and hath exerted itself.
'And I have been considering, and lo, a young he-goat hath come from the west, over the face of the whole earth, whom none is touching in the earth; as to the young he-goat, a conspicuous horn is between its eyes.
And it cometh unto the ram possessing the two horns, that I had seen standing before the stream, and runneth unto it in the fury of its power.
And I have seen it coming near the ram, and it becometh embittered at it, and smiteth the ram, and breaketh its two horns, and there hath been no power in the ram to stand before it, and it casteth it to the earth, and trampleth it down, and there hath been no deliverer to the ram out of its power.
'And the young he-goat hath exerted itself very much, and when it is strong, broken hath been the great horn; and come up doth a vision of four in its place, at the four winds of the heavens.
Also read Daniel 11:1-4
'And I, in the first year of Darius the Mede, my standing is for a strengthener, and for a stronghold to him;
and, now, truth I declare to thee, Lo, yet three kings are standing for Persia, and the fourth doth become far richer than all, and according to his strength by his riches he stirreth up the whole, with the kingdom of Javan.
And a mighty king hath stood, and he hath ruled a great dominion, and hath done according to his will;
and according to his standing is his kingdom broken, and divided to the four winds of the heavens, and not to his posterity, nor according to his dominion that he ruled, for his kingdom is plucked up -- and for others apart from these.
When Alexander died (check out the 5th Trumpet at the beginning of Revelation 9 by the way), he was not there to restrain his 4 generals - they were not bound anymore. A lot of people died as they fought over his kingdom.