God is very specific in His use of words, and every word is very important in the specific contextual application. The context of Gen. 6:4 is set in the background of Noah's flood, and presents to the reader God's reason and justification for the judgment He brought upon the wicked sometime around 1500 - 1600 AA.
The point of the first 7 verses of Gen. chap. 6 is the description of the wickedness of ALL mankind, and God's decision to destroy them. But, we should note some key words.
" And it cometh to pass that mankind have begun to multiply on the face of the ground, and daughters have been born to them,
2 and sons of God see the daughters of men that they [are] fair, and they take to themselves women of all whom they have chosen.
3 And Jehovah saith, `My Spirit doth not strive in man -- to the age; in their erring they [are] flesh:' and his days have been an hundred and twenty years.
4 The fallen ones were in the earth in those days, and even afterwards when sons of God come in unto daughters of men, and they have borne to them -- they [are] the heroes, who, from of old, [are] the men of name.
5 And Jehovah seeth that abundant [is] the wickedness of man in the earth, and every imagination of the thoughts of his heart only evil all the day;
6 and Jehovah repenteth that He hath made man in the earth, and He grieveth Himself -- unto His heart.
7 And Jehovah saith, `I wipe away man whom I have prepared from off the face of the ground, from man unto beast, unto creeping thing, and unto fowl of the heavens, for I have repented that I have made them.'" (YLT)
The word translated in the above verses as "man" and "mankind" is Strong's Heb. 120 "haadam" or "adam." The race of man stemming from the seed of Adam! Source: Biblehub
In vs. 1 the BDB assigns definition 2, "collective man, mankind."
The background is speaking of wicked men, wicked mankind. It includes three types of mankind: the fallen ones, the daughters, and the sons of God.
The word in vs. 4 translated in Young's as "fallen ones" and in the KJV as "giants" is assigned Strong's Heb. 5303 "ha nepilim", short defintion "Nephilim" which further defined as "'giants', name of two peoples, one before the flood, and one after the flood." Source: Biblehub
This is a transliteration, not a translation. Strong "named" a people in Gen. 6:4, which the KJV translated as "giants". The reason the KJV translated it as "giant" is because the only other time this word is used it also means a name.
"33 and there we saw the Nephilim, sons of Anak, of the Nephilim; and we are in our own eyes as grasshoppers; and so we were in their eyes.'" (YLT)
In their report to Moses, the spies that were sent into the land of Canaan described the giants that were living in the land (about 400 years after the flood), and they had two names: Nephilim, sons of Anak, or the Anakim.
"28 only, surely the people which is dwelling in the land [is] strong; and the cities are fenced, very great; and also children of Anak we have seen there." (YLT)
There were many names for the giants living in Canaan and they include the Rephaim (also called the Zamzummim and the Emim), the Amorite, the Anakim, and the Zuzim. All of these are described in various parts of the OT which I have detailed in a post at my site "Giants: Rephaim, Zamzummim..." at ShreddingTheVeil.
The name "Nephilim" was clearly used for the giants in Num. 33. So, the KJV translated it that way when it occurred in Gen. 6:4. But, there is a further consideration as the context of the scriptures must be taken into consideration.
Gen. chap. 6 was speaking of the wickedness of ALL mankind, not just some of them. Strong's concordance provides a clue with the root word of "naphal" which means a "feller, ie. A bully, or tyrant - giant."
Today, the word "feller" is used sometimes to refer to those who cut down trees (from the ancient noun "fell"), or various idioms for a rough or bearish man. In the Middle English, when the KJV was translated it was used as an adjective for a fierce, cruel, savage or destructive person, and is the Old French nominative for "felon" or wicked. (See here)
If we think about it for a minute, the scriptures are logically describing the "felons" or wicked men whose great height most probably made them arrogant enough to believe they could do anything they wanted and get away with it. If you were standing 6 ft or more taller than some other people, what are they going to do against you?
And, that is exactly what God told the Israelite before they went into Canaan in Deu. 9:1-2.
"`Hear, Israel, thou art passing over to-day the Jordan, to go in to possess nations greater and mightier than thyself; cities great and fenced in the heavens;
2 a people great and tall, sons of Anakim, whom thou -- thou hast known, (and thou -- thou hast heard: Who doth station himself before sons of Anak?)" (YLT)
Gen. 6:4 included the "giants" or very tall PEOPLE in the category of the wicked, but all mankind was under judgment in God's plan for the flood. And, that is why Young's translated "ha nepilim" has "fallen ones" because it was not just the giants that were wicked.
The verse also clearly states that they existed before the sons of God mated with the daughters of men. The words "even afterwards" are the indication of the prior existence. So the wicked fallen ones were not a result of the sons of God choosing to marry the daughters of men.
The entire Bible, both OT and NT work together. All of it is from one source, the Holy Spirit of God. He will not contradict Himself. So, if our understanding or use of one scripture contradicts another of His scriptures then we are misunderstanding it.
"12 So, then, brethren, we are debtors, not to the flesh, to live according to the flesh;
13 for if according to the flesh ye do live, ye are about to die; and if, by the Spirit, the deeds of the body ye put to death, ye shall live;
14 for as many as are led by the Spirit of God, these are the sons of God;" (YLT)
The NT defines the sons of God as those who follow after the spirit of God.
The context of the scriptures has to govern the meaning of the use of sons of God. In Gen. 6 God was displeased with everyone of the species of MAN except for the one who walked with Him, namely Noah. Noah was perfect, not because of his blood line, but because he walked with God (Gen. 6:8-9).
A very strong case where the sons of God are used for the heavenly angels is found in Job 1:6, 2:1, and 38:7 where it appears that God held a staff meeting in heaven.
A further consideration is that the angels in heaven listen to God and do His will (Psa. 103:20).
Moreover, we are told in Matt. 22:30 and Mark 12:25 that the angels in heaven do not marry. As marriage is the only lawful method for procreation, the meaning is that angels DO NOT PROCREATE!
Gen. 6:4 used the phrase "sons of God" because the Holy Spirit had just listed those men who followed after the Spirit of God in Gen. chap. 5! The men of the genealogy provided in chap. 5 were those men who looked upon the fair daughters of the wicked and began inter-marrying. That was the cause for all mankind to slip into a "fallen" nature.
Never ever does the Bible speak about a hybrid species of angels fornicating with women. That is false teaching from pseudepigraphal writings which have been thoroughly discarded by scholars as unscriptural. Yet, men have overlaid the mystical teachings that have evolved from Babylonian Kabalistic writings to try to "add" meaning to the word of God.
This has implications for many other "mystical" teachings that have been applied to God's word. I won't go into them here.
The fallen ones, or "giants" of Gen. chap. 6 were not heavenly angels. They were all of the wicked evil people.
As God is not writing a science-fiction novel, then we should be more logical in our reasoning. The giants were part of the race of Adam, very tall people. Their height was a factor of the DNA, just like eye color, hair color, and skin color. Just as today we have some people who are 7 or 8 ft tall. It is still in the DNA for mankind.