40 watts of fluoros isn't going to provide enough photosysnthesis for plants the whole way through. Fluoros work great for cloning, but once out of the seedling stage (4-6 nodes or branches) you need to get either a larger CFL (Compact Fluoroescent) or a HID light (High Pressure Sodium or Metal Halide) Neither are cheap. I had a cabinet at one time that was about your size, and used a 250 watt High Pressure Sodium light. It cost about 220 dollars (US Money). I abused my plants in all kinds of ways, high temps, too much training/pruning etcetera and managed to get 6 ounces out of the 3 plants. If you invest in a good light, and read all you can on this site, you should get 8-10 ounces depending on what strain you have. Make sure as well that you have good air circulation. Plants need constant fresh air for growth. Plants use CO2 quickly, and if it isn't replaced growth stops. Hope this helps.