You could get away with 3 for sure but 4 in full bloom is pushing it. Unless you have a short veg.
If it were me and time was of no issue I'd train 2 bigger girls and go that way but if you don't got time then throw 3 or 4 bitches in there and get the air moving
Sure you could do vertical if you he switched to HID, that'd really be the way to go. Drop yea a light in the center and put plants all the way around it