Being mentioned in the Bible does not mean it is lawful. Slavery in Biblical times is different than the abominable practices from 200 years ago. There are parts of the Bible that specify how slaves should be treated; these would be analogous to 21st century relationships between employer and employee.
Concubines were legal? In 1 Kings (since you said Solomon) it states:
“You shall not intermarry with them, nor they with you. Surely they will turn away your hearts after their gods.” Solomon clung to these in love. And he had seven hundred wives, princesses, and three hundred concubines; and his wives turned away his heart.
I Kings 11:2-3 NKJV
I Kings 11:2-3 from the nations of whom the LORD had said to the children of Israel, “You shall not intermarry with them, nor they with you. Surely they will turn away your hearts after their gods.” Solomon clung to | New King James Version (NKJV) | Download The Bible App Now
This seems rather as an admonishment (which was disobeyed) rather than a legalization.
As for Genesis 4, I kindly direct you to this article and Leviticus 18.