Genesis 4 is not merely the consequence of Genesis 3. It's a retelling of it. The stories have an echo that get louder every time.
In Genesis 3, we have:
Adam: A tiller of the ground -> fruit -> broken relationship -> a curse of death -> God's protection (covering) -> the man has a son.
In Genesis 4, we have:
Cain: A tiller of the ground, -> fruit -> broken relationship -> a curse of death -> God's protection (a mark) -> he has a son.