A man has 4 wives. Now he wants to marry the fifth, but he knows that he cant do so. So he is denying his first wife to make his fifth wife lawful in marriage, but actually he is still in wedlock with the first. The first wife is still claiming that she is still married to him. The two witnesses who were present at the first marriage are coincidentally two first uncles from mother's and father's side of the fifth wife. They are also denying ever witnessing the marriage. Is the marriage of the first wife valid?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

It is prohibited for a man to marry more than four women as Allah Says (what means): {…then marry those that please you of [other] women, two or three or four.} [Quran 4: 3]

Also for the Hadeeth that Imam Ahmad and others reported from Ibn ‘Umar that “Ghaylan Ibn Salamah al-Thaqafi accepted Islam and he had ten wives in the pre-Islamic period; the Prophet told him to keep only four of them.”

However, if a husband issues a revocable divorce and the waiting period of that wife has finished, or if he issues an irrevocable divorce, then he could marry another woman. Know that during the waiting period of a revocable divorce, this husband cannot marry another woman since she is still considered his wife and he can take her back before the end of the waiting period without having to conduct any new marriage contract, or give bridal money, or even get the permission of her guardian.

Therefore, if the mentioned person has divorced his wife an irrevocable divorce, or a revocable divorce but her waiting period has expired, then he can marry another woman; otherwise, it is forbidden for him and being with her is pure Zina (adultery).

Allah knows best.