>If Adam and Eve and Cain and Abel were the only people at that time, then who are those people in the land of Nod that Cain went to live with?
You started with a false premise. Adam and Eve and Cain and Abel were not the only people when he went to Nod. According to the Scriptures, men lived for centuries in those days, on average somewhere in the 900 year range unless they were killed first. Adam had other children born by the time Cain slaughter Abel and his tribe. Cain had a family as well, having married his sister. He took his wife and children with him. But even when he left, he didn't go immediately to the east (Mesopotamia). He wandered about for hundreds of years first, he and his tribe. They were the hunters and gatherers because Cain (and therefore his seed also) were cursed with the inability to grow crops. Only when they began pillaging the others, taking captives, and having slaves to till their lands, did he and his tribe settle in Mesopotamia to become the pre-flood people of Sumer.