What should I do regarding my first wife, who doesn't respect me after bearing four children, and my second wife, whom I've come to suspect due to her accusations that I don't satisfy her?

Doubts are from Satan, and one should not pay attention to them unless they are based on overwhelming evidence. The mere statement of the wife that she is not satisfied is not a justification for having ill thoughts about her. Rather, one should treat her kindly and be just between her and the other wife, giving each her due right, for the Prophet, peace and blessings be upon him, said: "The best of you is the best to his family," and "Whoever has two wives and inclines to one of them over the other will come on the Day of Resurrection with one of his sides drooping."