I forgot to give a few more details. I'm using General Hydroponics nutrient Flora Series, right now since the plants are so small I've started with 550 ppm (1 part micro 1 part gro 1/2 part bloom) as I get growth will bump up to over 1000 ppm.

18 hours light / 6 hours rest... pump running 24/7

Anybody see any problems? Please feel free to comment on my set-up.

I am growing lettuce under a 400 watt MH with a reflector and a glass cover on the enclosure. 4 feet seems a little far up though. I think you could safely move it down to about 2 feet. A good rule of thumb is to use your hand a a guide for how close to move your light source. Since lettuce likes a cooler environment, just make sure that your hand doesn't feel hot at the top of the plants. Gentle warmth should be ok. Would love to see some pics...

Thanks..

I lowered my light to within 24" of the top of plants... noticeable growth, great advice thanks

Here is a link that might be useful: First Try Lettuce

Very hi-tech! They will definitely like the more intense light.