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.