Invalidity of marriage to a fifth wife or more

Question 12: If a person is married to four wives and marries a fifth wife later and she gives birth to one baby, will the baby belong to this father? 

Answer: There is no doubt that marriage to a fifth wife is Baatil (null and void) as agreed upon unanimously by scholars (may Allaah be merciful with them). Al-Haafid Ibn Katheer (may Allaah be merciful to him) mentioned in his Tafseer (explanation of the meanings of the Qur’aan) that scholars, except Shiites, unanimously agreed that it is prohibited to marry a fifth wife… read more here.