Give us some specifics about your room and other gear. Are you using a projector? What size is your room? How many seats?
Phantom center works well, but anyone sitting way off axis will not get the "effect". The fours have a wide sweet spot, so depending on the specifics of your setup, two may work great.
I currently have my theater setup in my living room. I have a 50" plasma, and two couches, but will be remodeling later this year and do a DIY screen with projector and individual seats. I will probably have six seats as there is normally only three of us that watch movies together on a regular basis. My room is 19' x 22' x 10'.
Do you think two 4Pis will work while I use the plasma, and add a third when I do the projector setup? I don't foresee the seating positions ever being really wide (probably 2 rows of 3 seats). Maybe I won't even see the benefit of a center with narrow setup?