Your assumption that there were only 4, is merely a presumption unsupported by Scripture.
i go by what scripture says since GEN is in the beginning that is what i go by
So maybe the book doesn't tell us everything.
i have no doubt i am well aware of how reproduction happens chapter 5 gives us a sneak peak of tim 800 years 4 And the days of Adam after he had begotten Seth were eight hundred years: and he begat sons and daughters: backing up to chapter 2 : 7 And the Lord God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living soul. there was no other humans that we know of . chapter 2 . we see God instituted Marriage man and wife
21 And the Lord God caused a deep sleep to fall upon Adam, and he slept: and he took one of his ribs, and closed up the flesh instead thereof;
22 And the rib, which the Lord God had taken from man, made he a woman, and brought her unto the man.
23 And Adam said, This is now bone of my bones, and flesh of my flesh: she shall be called Woman, because she was taken out of Man.
24 Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh.
backing up to chapter 1 verse 28 the marching orders
27 So God created man in his own image, in the image of God created he him; male and female created he them.
28 And God blessed them, and God said unto them, Be fruitful, and multiply, and replenish the earth, and subdue it: and have dominion over the fish of the sea, and over the fowl of the air, and over every living thing that moveth upon the earth. (Be fruitful, and multiply )
chapter 4 thy figured out how they birds and the bees work lol
And Adam knew ( sexual relationship } Eve his wife; and she conceived, and bare Cain, and said, I have gotten a man from the Lord.
2 And she again bare his brother Abel. And Abel was a keeper of sheep, but Cain was a tiller of the ground.
what is sad is soon the first murder took place. over jealousy which imo is a carryover from the fall in the garden . 3 sins took place which amounts to not listening to God the lust of the flesh, the lust of the eyes, and the pride of life all from hath God said . he entertained the thought, by planting a seed of doubt. once again many things are hard to understand . but the fall still affects us today .the good news is found in john 3:16 and 2 cor 5:21 -> he became sin ( a sin offering ) who knew no sin Christ made a way for us to be free from fall in the garden . as paul harvey use to say and know you kno the rest of the story