Yeah, what Tank and Labby said. I voted for "four" -- I would have voted for "five" if that were a separate choice, but I can't see any way they'd go as far as 7-8 seasons.
Frankly, from what I'd heard about the planned fifth season, I think I'm grateful they stopped where they did. Y'know, before they could screw it up any further... but then I've often complained about poor writing on this show. The premise and potential were fabulous, but the execution was... well, let's just say I love episode rewrites, and leave it at that.
PJ