That's a tuff one. If you are limited with placement its a real crap shoot if the placement will be good or not. How are you 4Pi's placed and are the sub woofer placements in the same plane along the same wall??? How low are you crossing over?

You should crossover as low as possible. If the sub and mains are on the same wall then it should be much harder to hear the sub provided you cross low enough.

Can you have a friend lend you a sub to get an idea about the placement. If it's a bad spot in the room it won't matter what you put it there so that might save you some time and heart ache. Get yourself a good set-up CD that has sweeps for sub integration and borrow an SPL metter if you can. If you can get your friends or a dealer loaner to integrate well you should be fine.

Good Luck