Depends on what you want to pay for it.

Obviously the value of a 4 letter domain depends on the letters and the buyer.

For example:

Loan.com would be far more valuable then DQWE.com

On the other hand if the buyer has a company named Deluxe Quantum Widget Experts then they might deem the domain DQWE to have some value over what the average potential buyer might be willing to pay for the 4 letter domain.

With all the variations of extentions and changes coming in 2009 its hard to put a value on .com generally. Specifically the value of a .com is mainly impacted on keyword, relavance to specific industry, ability to brand, meaning, and other factors, after all that - the shorter the better.

There are services that can help appraise domains such as sedo.com.  I would never base a business decission on an appraisal from sedo or other services like it, but it can give you a general indication of value (from a ball park perspective).