I mean the length of the bulb. Is it 48 inches as well? And does it look like it is 1 inch or 1 1/2 inches in diameter?
This is why I'm asking--fluorescent tubes of any particular size are always going to be the same wattage as any other of that size. They come in different color temperatures etc, but always the same wattage.
A 48 inch bulb that has a diameter of 1 inch (called T8), is going to be 32 watts, and a 48 inch bulb with a diameter of 1 1/2 inches (called T12), is going to be 40 watts.
When you say it is moonlight color are you saying you have a blue bulb that looks dim, not a daylight bulb? If thats what you have its almost like not having a light at all as far as your plants are concerned.
Over a 4 foot tank you are probably going to want to have two 48" T8 bulbs in order to grow anything but java fern. It would still be considered a low light tank.
HTH.