I reg'd one in .net that I had planned on using at one time. It does get some traffic, like maybe 1-3 a day even for a .net, with com/org are taken. If you find ones with commonly used acronyms, they will eventually be useful if you hold them long enough. You can sometimes also find some where the com is available with net or org taken and pick up some stray traffic.
Mine is GCDC.net which has lots of possiblies for acrnoyms like these examples I've made up:
Greater Cleveland Dive Club
Greater Cincinnati Data Corp
Government Contract Data Center
Golf Course Development Company
Gulf Coast Delivery Company
Glenwood Canyon Development Committee
Germantown Community Development Council
Green County Data Center
I think the key with 4 letters is to find very versatile and popular two letter combos and mix them.They won't be as hot or valuable as 3-lettes, but could still be profitable.