Montana is correct.
I'm sure you know this, but in case you don't...remember the original Word of God did not have chapters as we know them today.
Man came along later and divided the Bible into chapters and verses for easy reference.
That being said...Genesis 2:1-3 and the first half of vs. 4 actually belong at the end of Genesis 1. So here's how the Bible should've been divided:
Genesis 1 should've ended like this:
And God saw every thing that he had made, and, behold, it was very good. And the evening and the morning were the sixth day. (Gen 1:31)
Thus the heavens and the earth were finished, and all the host of them (Gen 2:1)
And on the seventh day God ended his work which he had made; and he rested on the seventh day from all his work which he had made. (Gen 2:2)
And God blessed the seventh day, and sanctified it: because that in it he had rested from all his work which God created and made. (Gen 2:3)
These are the generations of the heavens and of the earth when they were created.(Gen 2:4)
Gensis 2 shoud've began like this:
In the day that the LORD God made the earth and the heavens, (2nd part of vs 4)
And every plant of the field before it was in the earth, and every herb of the field before it grew: for the LORD God had not caused it to rain upon the earth, and there was not a man to till the ground. (vs. 5)
But there went up a mist from the earth, and watered the whole face of the ground. (vs. 6)
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. (vs. 7)
....and so on and on...
Genesis 2 is a going over certain aspects of Genesis 1 to bring about certain details that have not been given to us yet. The "Day" is not about Adam and Eve...it's about the day that God created the heavens and the earth.
Also, your theory on Genesis 4 can be solved by simply understanding that Adam and Eve had to have other children besides Cain, Abel, and Seth...so their part of the world may have had hundreds of people by the time Abel sinned.