Muslims are allowed to have more than one wife simultaneously in The Quran. Following is the translation of the relevant verse by MAS Abdelhaleem:
"If you fear that you will not deal fairly with orphan girls (Yatama), you may marry whichever [other] women (Nisa) seem good to you, two, three, or four. If you fear that you cannot be equitable [to them], then marry only one, or [marry] your slave(s): that is more likely to make you avoid bias." (Quran 4:3)
The problem with this translation (as with most translations) is that the only word given in this verse is 'Yatama', which means 'orphans' [no gender implied] and not 'orphan girls'. The women mentioned (that you can marry) are most likely the mothers of the orphans and not the orphans (girls) themselves.
Some people say that this verse allows believers to marry orphan girls as this will be a kind of support for them; with marriage orphan girls will receive food, shelter and a family from their husbands. This does not seem to be a correct interpretation. First of all, the Arabic verse only says 'orphans' and not 'orphan girls'. Secondly, if the orphan girls get support by marrying then what is the support mechanism for the orphan boys? Or their widowed mothers?
Based upon this logic, we can infer that it is not the orphan girls who should be married but it is their mothers (Nisa – Adult women) who should be married; this way all persons who became helpless due to the death of a person – the widow, orphan sons and orphan daughters – will get the support.
So, does Quran allows marriage to orphan girls? Not if they are under-age. Yes, in another verse Quran does allow marrying orphaned females; the word it uses is 'orphan (grown) women' and calls them 'Yataman-Nisa'; not just 'Yatama'. Here is that verse:
"They ask you [Prophet] for a ruling about women. Say, ‘God Himself gives you a ruling about them. You already have what has been recited to you in the Scripture about orphan (Yatama) girls (Nisa – Women) [in your charge] from whom you withhold the prescribed shares [of their inheritance] and whom you wish to marry, and also about helpless children –– God instructs you to treat orphans fairly: He is well aware of whatever good you do." (4:127)
In (4:127), an obligatory treatment of kindness is prescribed when dealing all orphans; whether orphan children or orphan women whom a person wants to marry. Thus, the above verses allow that you can have second, third or fourth wife, but only when you want to support the orphans.
The correct procedure is to marry the widowed female and then support the whole family. More background is provided by Riffat Hassan, Professor Emerita at University of Louisville. She writes that during the early days of Islam, as a result of wars, a large number of Muslim men were killed. This resulted in a large number of widowed women and orphaned children. In order to support them and protect their property, Allah (SWT) revealed (4:2), the verse preceding to the 4 marriage verse (4:3):
“Give orphans their property, do not replace [their] good things with bad, and do not consume their property with your own –– a great sin.” (4:2) Taken together, verses (4:2 - 3) limit more than one marriage only during the times of great social hardship and for humanitarian purposes only.
Accordingly, marrying more than one women is not a right, its not a privilege, but its a responsibility that can be practiced when society demands for it.
There is only one exception, that is The Holy Prophet Muhammad (PBUH) himself. He married his first wife when he was 25. The marriage lasted for 25 years until his first wife passed away. Only after that, at the age of 50, The Prophet married other women. Most of these women were old and widowed. Every instance of marriage was allowed by Allah (SWT) to set some sort of example, for example, for social or political purposes & to strengthen family and tribal bonds. He was given this special permission in Quran itself:
"This is only for you [Prophet] and not the rest of the believers" (33:50).
But, after that verse, after his last marriage, even The Holy Prophet (PBUH) was prohibited to have any more wives:
"You [Prophet] are not permitted to take any further wives, nor to exchange the wives you have for others..." (33:52).
Therefore, after a certain number of social examples were set, Allah (SWT) asked The Holy Prophet (PBUH) also not to marry another woman. In the end, we reiterate again that a 2nd, 3rd or 4th marriage is allowed only when the intent is to help someone who is in need of help.