In most cases the lights (cfls) should be about 3 inches above the plant. That way the plant doesnt become too stretched out. The closer the light the more full bodied the plant will be. But you have a 50watt HPS? Usually you see higher HPS lights (200 watt?)
How often do you water them?
At 4 weeks, depending on what type of soil you have, the nutrients are all used up. So you might want to start feeding them at 1/4 manufacturers strength and gradually work your way up to full dose.
Have you tested PH of soil?