I'm not for sure about the number of plants, but a 400 watt should cover a 4x4 or 5x5 space. But of course more light is almost always a good thing.

I have my HPS in a cool tube, and since we only have a really weak fan right now, the room stays around 85 during the day which is pretty high for flowering, I want to get it down below 80 so hopefully the new fan should do that. Right now I don't even have an exhaust fan per se, its just a regular round fan that is ghetto rigged on to some ducting pulling air through the tube and out the room, but it works alright. Keeps the tube cool enough I can put my hand on it comfortably when its been running all day. I dont know about summertime though, we started the HPS in october, but it was still like 90 degrees outside. So far, no matter what the temp outside, our room has stayed the same, right in that 85-83 degree range. We have a whole lot of circulation in the room with a big ass fan and a few oscilating fans, so it feels cooler to me than the thermometer reads. It felt way worse before we got the big fan to circulate air.

But yeah, I'm an official LST whore now, i recommend it to everyone simply cause I can't believe how awesome my girls turned out. Its amazing to me how simply tying a plant sideways can change the shape it grows so much.