You have 4 tables, but for this tournament would you expect that you would use all 4? The way you phrase it it sounds like you'd expect 15-20, am I reading that correctly?
Let me get that information and I will reply again with more details about structure. If you're going to have 20 or less players you're not going to have a very good "shootout" structured tournament. Generally only the winner of each table makes the next table, but I think if you could get 40 then each table should play down to 2 and then reform at a final table of 8. I suppose it would be kinda fun to have 2 tables play down to 1 on each and then those two winners play heads up, too.
I like that even though you have a small room, you are still trying to keep things fun and interesting.