Why does Islam limit plural marriage to four wives and no more?
Question

                She wants to know why does Islam restricts the number of marriage to 4? Why is it not 3 or 5?
Praise be to Allah.
    Firstly: 

    The Muslim must submit to all the rulings 
    of Allah, both commands and prohibitions, whether it is in accordance with 
    his opinion or not, and whether it suits his whims and desires or otherwise. 
    If the wisdom behind it becomes clear to him – whether the matter is 
    prohibited or permitted – that increases him in faith and submission.

    If the wisdom behind it is not clear to 
    him, he accepts it and submits, with certainty that Allah, may He be 
    exalted, does not prescribe anything except for a good reason, and this 
    reason or wisdom may be hidden to some people, or some people may know some 
    of it. 

    Allah, may He be exalted, says 
    (interpretation of the meaning):

    “But no, by your Lord, they can have no 
    Faith, until they make you (O Muhammad SAW) judge in all disputes between 
    them, and find in themselves no resistance against your decisions, and 
    accept (them) with full submission”

    [an-Nisa’ 4:65]. 

    Al-Bukhaari (321) and Muslim (335) 
    narrated that Mu‘aadhah said: I asked ‘Aa’ishah: Why does a menstruating 
    woman have to make up missed fasts but not missed prayers? She said: Are you 
    a Harooriyyah (Khaariji)? I said: I am not a Harooriyyah, but I am asking. 
    She said: That used to happen to us and we were commanded to make up the 
    fasts but we were not commanded to make up the prayers. 

    See also the answer to question no. 
    50330. 

    Secondly: 

    Plural marriage is something natural that 
    is known in all nations, although they differ in the ways in which it is 
    applied. When man-made laws prohibit plural marriage, people resort to 
    having illicit relationships outside the home. Plural marriage was known 
    during the Jaahiliyyah, then Islam came with that which is suited to sound 
    human nature, so it put limits on what the people were doing and imposed 
    some guidelines. 

    Thirdly: 

    This is a divinely ordained law, and 
    divinely ordained law is wise in and of itself. There is no need for the 
    accountable individual to know the reason and wisdom behind it; rather what 
    the accountable individual is required to do is submit. 

    One of the reasons behind this law is to 
    test people. At the beginning of Islam, the qiblah (direction faced in 
    prayer) was Bayt al-Maqdis (Jerusalem), then Allah abrogated it and 
    commanded the people to face towards the Sacred House (the Ka‘bah, in Makkah). 
    As for the sincere believers who were certain in their faith, they said “we 
    hear and we obey”, but the disbelievers and hypocrites in whose hearts was 
    doubt said: “What has turned them (Muslims) from 
    their Qiblah (prayer direction (towards 
    Jerusalem)) to which they were used to 
    face in prayer?” [al-Baqarah 2:142]. 

    Opening the door to argument in such 
    matters leads to doubt concerning sharee‘ah and its rulings. Then someone 
    will say: why are the prayers five and not six? Why is Zuhr prayer four 
    rak‘ahs and not eight? Why is the month of Ramadan singled out for fasting 
    to the exclusion of others? If it is said that it is because Qur’an was in 
    that month, they may say: why was Qur’an revealed in that particular month? 
    Why was it not revealed in a sacred month, because the sacred months are 
    venerated? And so on, without end. The one who says “we hear and we obey” 
    puts a stop to that altogether. 

    Allah, may He be exalted, says 
    (interpretation of the meaning): “And We made the Qiblah (prayer 
    direction towards Jerusalem) which you used to face, only to test those who 
    followed the Messenger (Muhammad SAW) from those who would turn on their 
    heels (i.e. disobey the Messenger)” [al-Baqarah 2:143]. 

    When the command comes, with what Allah 
    enjoins or forbids, one should say: “we hear and we obey.” If the wisdom 
    behind it is clear to him, or is partially clear, that increases him in 
    faith and submission, otherwise he should continue to hear and obey, because 
    he knows that Allah does not prescribe anything except there is great wisdom 
    behind it. 

    Allah, may He be exalted, says 
    (interpretation of the meaning): 

    “The Messenger (Muhammad SAW) believes 
    in what has been sent down to him from his Lord, and (so do) the believers. 
    Each one believes in Allah, His Angels, His Books, and His Messengers. They 
    say, ‘We make no distinction between one another of His Messengers’" - and 
    they say, ‘We hear, and we obey. (We seek) Your Forgiveness, our Lord, and 
    to You is the return (of all)’” [al-Baqarah 2:285]. 

    “It is not for a believer, man or 
    woman, when Allah and His Messenger have decreed a matter that they should 
    have any option in their decision. And whoever disobeys Allah and His 
    Messenger, he has indeed strayed in a plain error”

    [al-Ahzaab 33:36]. 

    Please see also the answer to question no.
    14022. 

    And Allah knows best.