Hey there gangI have one of those 20w 24" grow lights, I only have one plant growing it's doing ok but I added 2 75w cfl's because the 24" tube only puts out about 1200 lumens, I think there doing better now plus there really is no added heat with cfl's. from what I am reading the more light the better just watch the heat, I dont think 2 of those 20w are enough for 4 plants but I am no pro.