a 1000 watt hps is more than enough for four trees. They will be very nice. I suggest you take advantage of the penetration power of a 1000 watter and let them go big. cool. I just moved a rapid rooter into a five gallon container with a 3 in netpot with recycled tire mulch as the medium. I filled the water level up very high, to half the net pot and at the point the roots were just out of the rapid rooter. They are very hard to overwater. Not like rockwool. It grew fine and in under a week the roots were down past the netpots bottom and I let the water lower.