Technically, yes. However, it is not a rule or a commandment to
have more than one wife, but rather, it is considered as an
exception.
Few Muslim men actually take more than one wife for the obvious
reasons of lack of resources to support more than one wife, not
being able to treat them equally, and so on. While the Quran does
say that up to four wives are allowed, the practice is discouraged
(pay attention to the words in bold):
'.... Marry women of your choice, two, or three, or four;
but if you fear that you shall not be able to deal justly
(with them), then only one....'
- Surah An-Nisa (Women), 4:3.
Later on in the same surah:
'... You will never be able to be fair and just among women
(wives)...."
- Surah An-Nisa (Women), 4:129.
To sum it up, while having four wives is allowed in Islam, it is
discouraged and there are conditions attached to it, i.e. they must
all be treated equally both physically and emotionally. For
instance, if one of them has a room to herself all the other wives
must have their own rooms as well. There is no such thing as a
'favoured wife'.
So in the end, you don't see a lot of Muslim men around practising
polygamy.
Resources:
http://www.islamonline.com/cgi-bin/news_service/spot_full_story.asp?service_id=795