400w is perfect for 4 plants. You don't need supplemental light unless your plants are going to grow taller than the throw of a 400w bulb (around 2'. So if the pot is 10" and the bare stem is 10" you're good to go for a 44" plant.All I have is my 400w MH. Would that be enough to flower my 4 plants I have veggin? They are all about 3 feet tall.