What are your growing conditions like? Did you have heat issues in the 4x4 with the two lights going? If not then you'll be fine in the 4x8.
My grow room is in a cold basement/crawl space so with only one 400W HID going my heater has to kick in once in a while to keep the temp at 75 even during the summer. It takes two of those or a 1000W going to get the exhaust fan to kick in from too high a temp.
I'm vegging some plants under a 400W CMH in a spare bedroom now and with summer on the way it's already hitting 80F in there with the window open. I don't have an exhaust fan in there so it's passive ventilation. Impossible to keep my RH up over 30% even with the humidifier going as long as the window's open. It's only 14% here in the mancave atm.
You should get better yields with the plants spread out a bit.