Monogamy and Polygamy
It is a misunderstanding that Islam has permitted polygamy (to marry and keep four wives at a time). The fundamental principle is to have one wife at a time. The Quranic instructions that if one decides to marry another woman in place of his existing wife he has to divorce (as per the given procedure) and this itself clarifies that no one is allowed to have more than one wife at one time. (4:20) This should not lead to think that whenever a person wants to marry another woman in place of his existing wife, he might divorce her and go for another marriage (for sex enjoyment or for other reasons). Second marriage is not permitted until and unless differences between husband and wife have reached the climax, ending up by declaring talaq (divorce) and the wife is divorced
To have more than one wife, up to a maximum of four at a time has been permitted under extra-ordinary circumstances and for valid reasons. For example, as a result of war when a considerable number of women became widows including those of very young age, and large number of children become orphans, and when their economic, social, ethical, and sexual problems cannot be solved by any other satisfactory means, the Islamic State can allow an exception to the general rule of monogamy. Such an exemption has to fulfil two conditions viz. (I) such a person will have to do justice and give equal treatment to all his wives, and (ii) he should be economically sound enough to bear the burden of such a large family. Of course consent of the first wife in this matter is necessary. If anyone of these conditions is not fulfilled, permission to go for next marriage is not granted and one should stick to the rule of one wife (4:3); this is the only verse found in the Quran on this subject. Rather at another place the Quran says that despite his strong desire it would not be possible for a husband to be fair and just towards women (wives). One should not be inclined wholly to one wife leaving the other in a state of suspension, like one who remains neither divorced nor undivorced (4:129).
Adapted from: "Family Life in Islam" by: "Dr. Mir Mustafa Hussain"