According to Sa'id bin al-Musayyib, there is no harm in a person marrying a fifth woman after giving an irrevocable divorce to his (fourth) wife, whether the (divorced woman) is pregnant or not.
Hadith Referenceسنن الدارقطني / كتاب النكاح / 3826
Hadith Takhrij«أخرجه الدارقطني فى ((سننه)) برقم: 3826 ، انفرد به المصنف من هذا الطريق»