Ruling on marrying a fifth wife during the ‘iddah following divorce of the fourth wife
Question

                If a man has 4 wives and he divorces one of them,is it permissible for him to marry someone else during this iddah period? What is the daleel,if the answer is no it's not permissible for him to marry someone during the iddah period will that last marriage be valid and what should be done to rectify the situation according to the quraan and sunnah.
Praise be to Allah.
    If the divorce that has been issued to the fourth wife is a 
    revocable divorce – i.e., it is the first or second talaaq – then the 
    scholars are unanimously agreed that a woman who has been given a revocable 
    divorce is still to be considered a wife until her ‘iddah is over. (al-Mughni, 
    7/104).  If it is proven that she is still your wife, then you should note 
    that the scholars are agreed that it is not permissible for a free man to 
    have more than four wives at one time, because of the hadeeth narrated by 
    al-Tirmidhi (1128) from Ibn ‘Umar (may Allaah be pleased with him) that 
    Ghaylaan ibn Salamah al-Thaqafi became Muslim and he had ten wives during 
    the Jaahiliyyah; they became Muslim along with him, and the Prophet  
    (peace and blessings of Allaah be upon him) commanded him to choose four of 
    them. This was classed as saheeh by al-Albaani, as stated in Saheeh Sunan 
    al-Tirmidhi, 1/329. 

    It is clear from the above that it is not permissible for a 
    man to marry a fifth wife during the ‘iddah of a wife who has been given a 
    revocable divorce, because by doing so he will have five wives at the same 
    time, and the Sahaabah, the four imams and all of Ahl al-Sunnah 
    wa’l-Jamaa’ah are agreed, in word and deed, that it is not permissible for a 
    man to have more than four wives at one time, except for the Prophet  
    (peace and blessings of Allaah be upon him). Shaykh al-Islam Ibn Taymiyah 
    said in al-Fataawa al-Kubra (4/154), that the great Taabi’i ‘Ubaydah 
    al-Salmaani said: “The companions of Muhammad  (peace and blessings of 
    Allaah be upon him) did not agree on anything as much as they agreed that a 
    fifth wife cannot be married during the ‘iddah of the fourth, and a woman 
    cannot be taken as a co-wife with her sister.” Whoever wants to have more 
    than four wives is going against the Book of Allaah and the Sunnah of His 
    Messenger Muhammad  (peace and blessings of Allaah be upon him), and is 
    going against Ahl al-Sunnah wa’l-Jamaa’ah. 

    See Fataawa al-Lajnah al-Daa’imah, al-Fataawa al-Jaami’ah 
    li’l-Mar’ah al-Muslimah, 2/641. 
If this has taken place, 
    then the marriage contract is invalid, and you have to leave her until the 
    ‘iddah of your divorced wife is over. If you have consummated the marriage 
    with her – i.e., with the fifth wife – then you have to give her the mahr 
    that is given to women of her social standing, and she has to wait out the 
    ‘iddah of one who is divorced. Then if you want to marry her after that, you 
    must make another marriage contract with her, meeting all the conditions 
    prescribed in sharee’ah for a marriage contract.

     But if your divorce of the fourth wife is an irrevocable 
    divorce – i.e., a third talaaq – then there is a difference of scholarly 
    opinion as to whether it is permissible to marry a fifth wife during the 
    ‘iddah of the fourth wife following a third talaaq. The Hanbalis and Hanafis 
    say that this is not allowed, and this is the view regarded as most correct 
    by Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allaah have mercy on him). See 
    Kitaab Fataawa al-Talaaq by Shaykh Ibn Baaz, 1/278. On this basis, the 
    ruling is the same as that described above in the case of a revocable 
    divorce. When the ‘iddah of the woman who has been divorced for a third time 
    comes to an end, it is then permissible for him to make a marriage contract 
    with her (the fifth woman). And Allaah knows best. 

    But in the case of the fourth wife dying, it is permissible 
    for him to get married after she dies, because the marriage is not 
    considered to be still in effect in this case.

     And Allaah knows best