with a 400 watt light I would leep the plants under 3 foot you will not get good enough light penatration for any taller of a plant. If you had better lighting then yes a 4-5 is going to yeild more but with less light all the bottom will be scragly shit that is pretty useless for anything other than hash. I have always found that in any given sq footage if fully covered in canopy whether you have 5 bigger plants or 10 smaller plants it will yield pretty much the same it all really depends on your light.