I like using
http://www.theitdomain.com/whoisloop.php to find domain names. If you fix the middle letters to be a vowel as second letter and the third letter being one that goes well in the middle of a word (e.g. r,l,w,y) and the outer letters being "Any", you may find a four letter domain that looks like a nice word. Also fixing two letters to be the same and the other two being "Any" then you may find a nice four letter .COM with two of the same letters. Rhyming letters in a 4 letter .COM are good as well. (e.g. C,D,E,T,V,B,G) It's also good to take into consideration that some letters are better than others.
One thing about using this is that you might find some four letter .COMs that are already registered that are listed as not being registered. Usually they are the names that sound to good to be true and so if you find a name that you like, you should double check it to avoid disappointment.
If your sick of looking, I don't think that these are registered:
SULR.COM RULJ.COM QULM.COM OULF.COM OULB.COM OULC.COM
NULQ.COM DULJ.COM HULX.COM JULD.COM KULX.COM LULD.COM
LULP.COM CULQ.COM AULX.COM PURB.COM
AULX.COM is the probably the best.