The general rule of thumb is .5grams/watt under ideal conditions, no matter how many plants ya have in there. Also, do ya have Sativa's or Indica's (or a mix, for that matter)? How's the ventalation?
Personally, I would go with 3-4. If anything, it'll give ya a good idea of what to expect next time without really hurting yer plants...


Fly High!