Why In Islam men are allowed to marry four women?

The issue of polygamy is a necessity dictated by the conditions of life. Polygamy is not newly legislated by Islam; rather, Islam came to find it unrestrictedly practiced without regulations and in inhumane manner. It is the religion of Islam that regulated and refined polygamy and made it a remedy for some urgent cases from which the community is suffering. Islam came to find men unrestrictedly allowed to have ten, more or less wives. Therefore, Islam restricted the permissible number of wives to four provided the man will treat them justly while possessing the financial ability to provide for them equally. Otherwise, It is obligatory for the man to restrict himself to one wife as per the words of Allah the Almighty, "Then marry those that please you of [other] women, two or three or four. But if you fear that you will not be just, then [marry only] one." (Qur'an 4: 3).
Polygamy in Islam was also legislated to resolve existing social problems relating to men, women and the entire society. Polygamy helps the man whose wife is barren or suffers from a disease which prevents the continuation of marital relationship between them. In this case, instead of divorcing his first wife, it is allowed for the husband to take a second wife to fulfill his physical needs and at the same time his first wife will continue to live in dignity and protection in her marital house which is a kind of excellence (Ihsan) and respect towards her.
Polygamy likewise resolves the problem of the divorced women or widows who seek chastity and safety under the protection of a husband who provides for her needs, even if she is a co-wife. Polygamy can help in resolving the problem of the entire society. When the ratio of women in the society exceeds that of men, according to statistics, especially after wars, it is better for a woman to maintain her chastity by being a co-wife rather than remaining unmarried and finally led to corruption and immorality. Despite the permissibility of polygamy in our time, the number of spinsters, widows and divorcees is increasing.