Is it permissible for a man married to four women, who has divorced one of them, to marry another woman during the divorced woman's iddah (waiting period)? What is the ruling on his marriage if he does so, and how can the situation be rectified according to the Quran and Sunnah?

If the divorce is revocable (first or second talaq), it is not permissible for the man to marry a fifth wife, because the revocably divorced woman is still considered a wife until her 'iddah (waiting period) expires. It is forbidden to combine more than four wives, based on the Almighty's saying: "Marry such women as seem good to you, in twos, or threes, or fours." If he marries a fifth wife in this situation, the marriage contract is void, and he must separate from her and pay the dowry of a similar woman if he has consummated the marriage with her. He can then remarry her after the 'iddah expires. However, if the divorce is irrevocable (third talaq), scholars have differed, but the preponderant opinion is that marrying a fifth wife during the 'iddah of an irrevocably divorced woman is prohibited until her 'iddah expires. In the case of the fourth wife's death, it is permissible to marry a fifth wife immediately.