Adam and Eve have babies, brothers Cain and Abel. Cain was a farmer, Abel was a shepherd. They both offer stuff to God, and God liked Abel’s, but didn’t much care for Cain’s, for no apparent reason...maybe God doesn’t like vegetables (in skimming ahead a bit, I am guessing that this is but the first of many, many examples of God arbitrarily playing favorites). So Cain gets jealous and kills Abel. God comes along and says, “Where is Abel?” (Again, not sounding so all-knowing at the moment. I thought God was supposed to know everything and be everywhere; this is the second time that people have done something while God is apparently not around, then God shows up and says “what’s going on, eh?”) God figures it out, and curses Cain to be a wanderer and a crappy farmer. Cain says he’s worried that people will kill him…I don’t know what he is worried about; the only people in existence are Adam, Eve, and himself. So God puts a mark on him so none of the mystery people will kill him.
Then it says, out of the blue, “Cain lay with his wife.” No mention whatsoever of who she was or where she came from. Again, at this point, the only people who exist (are explicitly mentioned) are Adam, Eve, and Cain. So either it was Eve, his mother; he had sisters that aren’t mentioned; or God went off and created some other people, who aren’t mentioned. Either way, it’s a bit weird.
The rest of chapter 4 and all of chapter 5 is a long and tedious genealogy from Adam to Noah.