After responding to Roy's comment in the post a couple below this one, I was curious what answers Google would bring to the question:
Where did Cain's wife come from?
After all, the Bible only talks about the creation of two original people, Adam and Eve. Now, they have more children after Cain and Abel, but when Cain wanders off to the land of Nod, after killing his brother, it talks about him "knowing" his wife and begatting lots of kids.
Some of the answers out there are pretty interesting...here's a sample:
I pulled up on Google:
God does not reveal everything to us in the Bible. He wants us to trust in Him and that is the most important message of the Bible. Many questions are not fully answered in the Bible, because they are irrelevant to us. The Bible does not say in detail where Cain got his wife. We do read in Genesis 5:4 that Adam and Eve had other sons and daughters apart from those that we read of in the Bible. The only answer we therefore have is that Cain married one of his sisters.
So don't worry yourself too much about Cain's wife. Cain and all his descendants died (Genesis 7:21-23). The important person to know is the Lord Jesus Christ.
It's not important, but he married one of his sisters, but that's not really important.
acknowledges the incest:
Since we don't know how old Cain was when he married, or how old his wife was, she could have been his sister, his niece, a cousin, or a more distant relative.
Here's an answer from a Catholic site
Regarding the details to your question, yes, there are some problems with the 4 people and where did Cain's wife come from. But these details are trivial compared to the deeper meaning of the story.
Wow! Good answer! That slakes my thirst for curiosity.
This Bible study page actually delves into genetics, though:
Cain married one of his sisters.
While such a marriage would certainly be very wrong today (as God later decreed, see the Fact Finder below), the rules were quite different in the early days of humanity. It wasn't a matter of Cain making a "wrong" choice, since it was his only choice in order to be able to obey God's command for humanity to "be fruitful and multiply" (Genesis 1:28).
In those early times, the defects and disorders that could later possibly occur in children from such marriages, from recessive genes, would not have been as likely, since recessive genes would have had little time to develop. The children could actually have been much healthier than humans are today, as evidenced by the far greater life spans of the earliest humans (e.g. Genesis 5:1-32).
God apparently created Adam and Eve with homogeneous DNA. Whew!
Anyway...lots of good answers out there. I'm glad I bothered to look. I'm totally satisfied now...aren't you?