If you can get a ounce per plant of your first grow then your doing ok.I would start flowerering at around a ft in height,but make sure that you keep the light's has close to the tops of the plants has possible.This is where most first time growers go wrong me included when i first started out.Also a 250 watt hps can get some nice weights but sometimes less is more 4 plants is about the max i would put under a 250.Also make sure that you use the biggest tubs that will fit into the grow room.You need to let the plant get a realy nice root mass,bigger the root mass the bigger the yeild.If you are struggleing to get 4 large tubs into the grow area then i would make a box up line it with pond liner.Make sure that you fill the bottom with some kind of drainage material.Also only water once a week there is no need to over water you will be doing yourself no favours by overwatering.The dryer a plant becomes the plant then sends out new roots in search of water,this is how to get a realy nice root mass.By doing these things you will easy reach your target of a ounce perhaps a ounce and harf.For your next grow try 2 plants instead of the 4 and flower that little bigger say 1 1/2ft to 2ft by doing this the plants will get more from the light that you are useing and you will get about the same ammount.