The wisdom in limiting the number of wives specifically to four:
"...This is from the completeness of His blessing and Shari'ah, and is in accordance with His wisdom, mercy, and benefit for His servants. This is because one of the purposes of marriage is sexual intercourse and the fulfillment of one's desire, and from the people are those who are overtaken by their desire, and are not satisfied with one. Therefore, He allowed him (the man) a second, third, and fourth one (i.e. three additional wives)...and the Legislator has attached numerous rulings to this number (three), as He has allowed the migrant to remain in Makkah for three days after completing his rituals, and allowed the traveller to wipe over his footwear for three days, and made the preferred length of hosting a guest to be three days..."
The wisdom in allowing multiple spouses for the man, in exclusion to the woman:
"...this is from the completeness of the Wisdom of the Exalted Lord, and His goodness and mercy towards His servants, and His taking their interests into account. He (Allah, 'azza wa jall) is Exalted above doing anything other than this, and His Sharee'ah (Legislation) is far from coming with other than this. If it was allowed for the woman to have two or more husbands, the world would be ruined, and progeny would be lost, and the husbands would end up killing each other, and a crisis would ensue, and the Fitnah (trials, tribulations) would become severe, and war would erupt. And how can the affair of the woman remain firm when she has bickering partners? And how can the affair of these partners themselves remain firm? That is why, the coming of the Sharee'ah with what it came with of opposition to this is from the greatest of proofs of the wisdom, mercy, and consideration of the Legisator.
So, if it is said, "How can the interests of the man be taken into account, with his being able to marry whom he wants, fulfill his desire, and move from one woman to the next in accordance with his desires and needs - all while the needs of the woman are just like his needs, and the desires of the woman are just like his desires?"
We answer: It is from the nature of the woman to be concealed behind the walls and hidden in the dept hs of her ho me, and her moods are cooler than that of the man, and her outer and inner movements are less than his movements, and the man has been given strength and intensity that leads his desire more so than with the woman, and he has been tested with what she has not been tested with, he is granted the choice of having multiple spouses that is not available to the woman. This is from what Allah has set aside exclusively for the male, and has preferred for them over women, just as He has preferred for them Messengership, Prophethood, Khilafah (Caliphate), kingship and leadership, governance, Jihaad, etc. over women. Also, He has made men to be guardians over women, traversing about for the purpose of looking after their best interests, constantly on the lookout for means of sustaining them, encountering dangers, and exposing themselves to all types of hardships in the path of doing what is best for their wives. Therefore, the Exalted Lord is Thankful and Forbearing, and He thanks them for this, and has compensated them by instilling for them what He did not instill for their wives.
And if you compare the fatigue, hardship, and efforts of the man in fulfilling the interests of the women with what the women have to endure of jealousy, you would find that the man's share of having to endure this fatigue, hardship, and effort is much greater than what the women have to endure of jealousy..."
"As for the one who says that the d esire of the woman is even more than that of the man, this is not the case. The driving force behind the desire is fervor (lit. heat). So, where is the fervor o f the woman in comparison to the fervor of the male?
...And from that which proves this is that if a man has intercourse with his wife, he can have intercourse with other women in the same time frame, and the Prophet used to go around to all of his wives in a single night, and Sulayman went to 90 women in a single night, and it is known that with each woman, he displays desire and fervor that drives him to have intercourse.
The woman, on the other hand, if the man fulfills his desire with her, becomes exhausted, and does not seek to fulfill her desire with any additional man within that time frame.
So, the wisdom of of the Decree, Legislation, Creation, and Command is implemented, and praise is for Allah."
Reference: I'laam al-Muwaqqi'een, 2/64-66.