buckworks - 1:11 am on Aug 15, 2012 (gmt 0)
Your best bet would be as Huskypup said, check out all the domain marketplaces, set some parameters for your search and just scour through what's available.
There's lots of four- and five-character domains available on the secondary market, but expect premium prices for those that could be considered to sound word-like and not, say, like a radio station call sign.
A caution: If you find a domain that you're thinking of using to build a serious brand, do diligent research about whether it might mean something in a different language and be sure you're comfortable with the meaning if it does.
Also, check whether you can get control of variations like the .com, .net, .org , your country's .tld and so on. The ability to acquire defensive registrations might make one domain more attractive than another.