Your question is VERY broad, not because you have asked a string of them, but because of what it points to.
The bull’s eye to hit, so as to be able to understand what is going on here isn’t 4 but 3.
The notion of God accomplishing ‘tasks’ by men in ‘unit periods of 3’ or ‘7’ is scattered throughout many parts of scripture, and after which 3s or 7s, continuation in that exercise takes on other elements like aspects of the 4th day in your query; or in the case of 7 days of the feasts, at the end of which on the eighth, something else was also scheduled to happen. This is the obvious structure.
Some famous ones being; 1) Abraham’s infamous journey of three days to the mountain during which he had to endure in the unthinkable thought. 2) the ultimate sacrifice in which messiah was 3 days in the heart of Sheol. 1 Peter 3:19–20
In addition there are these sporadic instances like,
A-Gen 40:12,
B-Exo 5:3,
C-Exo 10:22,
D-1Sa 9:20, the list is quite long;
which are all things spirit, and so there’s a signification to them in the element of 3.
To spare you details I will be brief, but within the context of the first string of your questions.
It's a broad concept that originates from the very nature of what it means that ‘Israel is the chosen nation by God’. Question; chosen for what?
Answer; to rule over all works of GOD in all creation, that honour going to all the firstborn males of Israel, but that would be on conditions of holiness and it would be an ages-long process for them to attain to this lofty duty.
Indicating that EVENTUALLY all these firstborns had to have a nature far above that of a human so as to be able to rule over both man and other beings that were higher in power.
At the time that Adam fell, dominion given him in Genesis 1:26-28 passed into hands that weren’t God’s original plan for His creation. It is sons of man that were created for this dominion, and so Israel was God’s choice to work with (Deut 32:8) so as to replace these spiritual entities that had interim dominion over God’s works, with Messiah as the Head over Israel, ref Genesis 3:15
Now the passing of this dominion from Adam into these entities’ care was under circumstances of rebellion, Luke 4:6, the Devil as the ringleader, and so while their dominion lasted in the earth, all their achievements would be marked as uncircumcised, this is the period of ‘three' years worth of produce or increase that shouldn’t be eaten from the field by Israel.
This is where comes in the famous element of the very first first-fruits that should be consecrated and brought to GOD in the fourth year.
Its the same principle in the tithe, and symbolic of the same rulers. An obscure pointer also is the fraction of 10 of all the increase. Ten alludes to aspects of dominion by the rebellious spirits in the earth in the time when messiah comes on the scene, Ref John 14:30 because ultimately Israel is about dominion in the earth.
This is why the number of horns(kings) on the head of the dragon is ten, denoting their dominion in the earth going way into millenium after the norm as we know it among men will have ceased; the same unit of 10 is in the 1000 years, to indicate the span that it takes for these ten kings to utterly be uprooted, and so during which period Messiah and His followers reign so as to uproot all their remnants of these ten kings that will still linger in the earth. Ref Dan 7:25-27 also here undertones of a 3 stick out. This is why I said your Qn was broad.
First-fruits of the field refer to aspects of the Firstborns who spiritually, and in the culture of Israel, are symbolic of rulers, and here, of God over all creation. Rev 14:4
This is the figure in Exodus about ‘killing off’ the firstborn of Pharaoh (the devil’s rulers) and setting apart the first born of Israel, man and beast, for Himself. Exodus 13:2.
Indicating that eventually these sons of Israel had to have a nature far above that of a simple human so as to rule over both man and other beings that were higher in power.
However, it’s after a very long time of detours and changes and delays because of man’s evil nature, that the Most High would achieve this in them turning them into the mighty ‘first-fruits’. These are they that have the first resurrection to rule with messiah, ref 1 Cor 15:23 and Rev 14:4.
So at the resurrection of these is the beginning of the ‘4th year’ when they only will be consecrated to the Most High for the duty of dominion that their errant father Adam forfeited to the Devil..Revelation 20:6
I hope that helps.