Maggie Koerth-Baker describes two interpretations of how Sharia Law applies to multiple marriages. First up, the view of Pakistani geopolitics researcher Azmat Hassan:
Sharia Law says a Muslim man can have four wives, but it comes with conditions. You can have four wives IF you love, care for, and treat them all equally. Up to, and including, having houses for each of them. The Koran is trying to make a point here, [Azmat] Hassan says, and it's not that having four wives is awesome. Instead, you're to understand that it's impossible to be that fair to multiple spouses at once, and thus, understand why you should only have one. This interpretation is common, he says. In fact, he claims it's a big part of why polygamy isn't particularly popular in Pakistan.
Islamic studies professor Mohammad Mahallati's interpretation:
When the Koran gives thumbs up to polygamy, he says, it's in the context of talking about how the community should care for widows and orphans. In a world where it was difficult for a woman to provide for herself—and where there were no formal social programs to fall back on—encouraging financially stable men to protect widows and orphans through marriage made sense.
The Dish 