A man is allowed to marry four women. Why can't a woman marry four men just like men? Woman is a human. She may need to marry more than one man.
The question is different than your comments, so I answer in two slightly different ways.
Why can't a woman marry four men just like men? Woman is a human. She may need to marry more than one man.
The "why?" question, does not have any essence to itself. You seem to already understand that it is forbidden by the religion. What you want is the reasoning, behind it.
For example: "Why did God impose a restriction to females to marry only one male?"
It is impossible for anyone to answer that question, due to the fact they are probably not God. Being Islam.SE, there is the structure of Islam which follows, meaning you source prophetic knowledge (Hadith) or God's word (Qur'an).
You could probably dig into the pathology and reason that the socioeconomic benefit at the time leads to such a system of a male marrying multiple females to be the beneficial thing to do on both sides; however, that is more of a anthropological approach, and not necessary God's reasoning (assuming there was such a thing), etc. There are principles you have to sort of assume, call it a leap of faith, or an axiom.
Also asserting that it may be a required need, for a female to marry multiple males, is slightly absurd, it is a relative problem. I cannot see how it would be a need. (It goes both ways, these are not needs, otherwise there is such a thing as exemption due to necessity.)
This is a philosophical conundrum, though I am not going to utter a damnation of equality. It is important to point out we are not all inherently equal beings in a physiological sense. Sexual dimorphism exists, thus leading to inequality. Inequality is not injustice, so thus the law which instills order, maybe validly (in a moral sense) different based on sex.
To extend deeper into the issue, there is Muslim opinion in which finds marriage in which the male has multiple wives fall under specific contexts, which maybe supported via anthropological evidences. Such as during times of war.
According to, "The Reliance of the Traveller" translated by Noah Ha Mim Keller says at page 530:
m6.10 It is unlawful for a free man to marry more than four women. It is fitter to confine oneself to just one.
Asking why women can't get 4 husbands is the same as asking:
- Why can't men get pregnant? (Isn't that "unequal"?)
- Why can't men also get a break each month from praying and other duties due to menstruation? (Isn't that "unequal"?)
- Why aren't women as physically capable as men? (Isn't that "unequal"?)
You can go on and on. The fact is the natural world is different for males and females. Islam does not pander to a post modern philosophical view of what is equality and what isn't. It deals with the real world, not empty ideals that are relative to some point in history.
In short, you're asking the wrong question.
If we are asking about why (in Islam) , we can easily say : because Islam says so. The whole concept of Islam is to adhere to the Islamic laws. If you are asking about the wisdom behind it , we can say things like : who's baby is this , as mentioned by Rebecca J. Stones, and the diseases , and of course we have : human nature , and other things . Equality is not superior in Islam .
If Islam differentiates between two things , then equalling them is not adhering to Islam. Besides , What kind of a feminine woman that would want to have many men as husbands ? ..
Ultimate equality is an illusion . (I mean in the physiological, emotional sense ) . I think she would be a masculine woman that hides behind shinny liberal terminologies , such as : equality , freedom , etc. For all I know, a woman may have 100 husbands , but why would she want to make her desires "islamic" ?