2 nice big ones should fill a 4X2 tent.
I use a 600 primary and suppliment with some cfl's in a 5X5 foot space.
Whole idea is to entirely fill your grow space with vegetation., so you can grow more number of plants with a lesser veg period, or veg them longer, growing larger plants till you switch them to 12/12.
End yield is about the same, just have your grow space 75% full of healthy foliage when you switch your lighting to 12/12