Why do some scholars say that polygamy is the exception and that the rule is one wife and other scholars say that the rule is up to four wives where polygamy is not necessarily an exception? Which one is it?
The Ideal is one wife and the permission to marry more than one is a an exception as can be inferred from the following verse:
Allah says, " And if you have reason to fear that you might not act equitably towards orphans, then marry from among [other] women such as are lawful to you - [even] two, or three, or four: but if you have reason to fear that you might not be able to treat them with equal fairness, then [only] one - or [from among] those whom you rightfully possess. This will make it more likely that you will not deviate from the right course." (Qur'an: 4:3)
Thus if there is a reasonable ground to suspect that he may not be able to do justice, then he is allowed to marry only one. It does not take much imagination to see that the stressful living conditions in the modern industrialized societies are not conducive to a plurality of wives; it is a fact that the vast majority of men are struggling even with one wife and children as they are unable to find enough time to give them the necessary, emotional and spiritual care that is essential for their development as responsible Muslims. In other words, life in the modern world has become so fast paced that it is next to impossible to do justice. This is why most scholars insist that we should keep to the ideal.
Moreover, we are also bound by the terms of our marriage contracts. In a milieu like ours where monogamy is the norm, one needs to get the permission of his first wife to marry another; for she had married him with the tacit understanding that she would be the only wife he will be having. So unless such an option had already been stipulated in the contract, he is not justified in marrying a second wife without her permission.