People chuckled at me in 2000 and 2001 when I was registering some of the better LLLL.coms (while you could still get some great LLL.coms for under $1500 each). I think the remaining LLLL.coms, today, are not very good (but still have potential), and it would be best, IMO, to pick up some of the better combinations from sellers for $25 to $200 (or more) each. Guage the popularity (potential demand) by looking at how many other extensions are gone and how they are being used, plus the number of Google matches for that acronym.
Common patterns are also something to consider. For example, LLLLs ending in "MG" could be for Media Group, Medical Group, Marketing Group, Management Group, etc. Names ending with "I" can be for Incorporated, International, Internet, etc. Endings in "A" could be for America or Association, etc. Endings in "C" could be for Company, Corporation, Consultants, etc. Endings in "FG" for Financial Group. Endings in "OA" can be for "Of America." ETC....