4 letter dot coms with no numbers or dashes seem do do pretty well. 4 letters that mean something or sound good are more valuable. A good example might be yast.com because most people can pronounce it, thus making it easier to remember. (though thats just an example off the top of my head) Another thing that makes it more valuable is if there are companies with that 4 letter acronym.
Personally I have seen 4 letter .coms that sell anywhere from $100 to $20,000. For those 4 letter dot coms that dont mean anything, are not a word, or are not pronouncable, I would say its only worth about $100 or $150.
The way I like to look at it is there are almost a half million combinations of true 4 letter domains. (456,967 that is) Thats a lot of domains, and not to mention 5 and 6 letter domains that are a word or mean something else all together.