Your surface area is 20 square feet. Then, you need to multiply by what ever the light density you wish to grow at. Example, many people use a 600 watt light for a 4x4 grow area. Many HPS bulbs put out about 89,000 lumens at 600 watts, so that equals roughly 5,500 lumens per square foot, or roughly 40 watts per square foot. The trouble with stating it in watts per square foot is that different lights are not as effecient at growing plants as others are per watt, nor are they even able to produce as many lumens per watt. If you can hit 5,000 lumens per square foot you should be OK in general. Think of this when you go looking at Home Depot for spiral FCL light bulbs, 37 watt bulb compares to X number (pick a number, 150 W, 300 W, 600 W...) of watts when compared to standard household bulbs... It is common sense that two, three, or even four small spiral bulbs will not power a 5 x 4 tent. You can get away with a lot less light in veg than flower, but you run the risk of taller thinner veg growth... If you want to keep the plants low to the ground, such as below 18 - 28 inches, which is what a 48 inch tall tent/box will force you to do, then you are asking for headaches if you have not thought through things like heat, planter/hydro system heights, bulb clearence from tops... Small tents mean you should consider florecent bulbs for sure, but even an HO bulb will produce heat which means higher humidity. This means you need to consider the turn over of air too, and that the air turn over should be constant. You have 80 cubic feet of air space that should be able to be vented within a ten minute time frame, as a rule of thumb. This means you can run a small 4 inch intake and small 4 inch vent fan continuously and it should work fine to help keep temps down, but you will still need air circulation inside your box/tent. Small fan(s) that pivot across the tops will stimulate stonger stalks and help keep plants lower. Higher lumens per sq foot will help keep plants lower. You should think about FEM pinching the tops method to control growth. Also consider the strain, it should be short in nature, like LR#2 seeds. Make sure to go right from veg to flower for your nutrients when plants hit 10 - 12 inches tall after you pinch the tops once (min.). Even with LR#2 you must keep temps down or they will never auto flower on even 16/8. So, to be safe make sure to drop cycle down to 10 on and 14 off for 2 1/2 weeks for your LR#2's and then, bump them back up to 14 on and 10 off when you see they are stating to flower. This will help ensure that you lower the flower stretch and kick in the cycle. You should make sure to run some blue light in flowering if you want to reduce the stretch. So, if you are using 48" HO tubes don't switch out all the blue bulbs for red/yellows when you switch to flowering. Make sure to follow the feeding schedule by dropping the nitrogen (N) and pushing up the (K) potassium when you switch to flowering. Watch your pH as it is vital, and make sure to use water without clorides. Letting some city waters stand for 24 hours may not always ensure clorides are out. Some forms they use will not evaporate off, so use distilled water, or Reverse Osmossis, water. R/O water is cheap if you get the stuff from the filtered water systems that are in many of the grocery stores. You can pay about 37 cents per gallon at Wallmart super store for R/O water, or 1.85 for a five gallon jug. Bring your own new clean gas can, or something, to put it in... Hope this helps you get started and thinking about this.
Bottom line, you may want to consider 4 bulb, or even 8 bulb, HO, or even a VHO florecent light with mixed bulbs changing them out with more blue for veg, then more reds for flower, but make sure to keep one, or two, blues for flowering in such a low tent/box. You should also have 2 fans for intake and exhaust, and you should consider 4'' low voltage types... Run them all the time and make sure to pinch your tops after a few (about 3 or 4) layers of leaves have grown on your preveg plants to keep them bushy. Put fans inside to move air and plants to keep them healthy, cool, short, strong... Just prop them up and blow them around and they will get stronger over a few weeks time... Stick a small blacklight strip light in there too to boost UV rays, why not, their cheap at Wallmart. UV helps kill molds and produce more heavenly plant oils/crystals... Yummy! GE makes a good germacidal bulb, but you can not turn them on all day like a lower powered black light, although that is the way to go, unless you can afford UV B high out put tanning type bulbs to blast them periodically. UV will dramatically increase potency to the levels that make small grows worth doing.
