Hot Water Heater

Im having a frustrating problem that perhaps somebody can help me solve: My hot water heater has worked fine for four years, but now, the water does not stay hot for very long. Its a 50 gal. Bradford-White. I have the thermostat turned up almost to scalding level and the water is very very hot when it comes out. However, it does not STAY hot for nearly as long as it used to. Any ideas would be greatly appreciated. Thanks a lot.