If Adam's first words about Eve in Genesis 2 echo God's heart towards humanity in Genesis 1, perhaps Genesis 4 is saying something: "And then Adam knew his wife."
After sin, God said: I will know them. I will experience them. I will suffer with them.
And then Cain kills Abel.
Perhaps God is saying to Abel: I'm going to suffer your senseless death.
Perhaps God is saying to Eve: I'm going to suffer your heartache and loss.
And perhaps God is saying to Cain: I'm going to suffer being rejected and hated.