How can one deal with a father who threatens his four wives with divorce over trivial matters and treats them unequally, knowing that he refuses to accept advice from his family members?

If a father oppresses his wives or abuses them, he must be gently and respectfully admonished against it, and this is not considered an act of disobedience (ʿuqūq). Imam Ahmad said: "One should enjoin good upon one's parents and forbid them from evil, teaching them gently, without harshness or abuse." If the son fears his father will reject the advice, he may seek help from righteous people to advise his father.