How do I replace a belt on an upright kirby?

First, go to a vacuum store ("sewing and vacuum" shop in some places) and purchase the correct belt. You unlock the rug plate by unhooking two latches on the rear of the nozzle. The rug plate is the steel item which rubs on the carpet and locks the brush roll in place. Just look on the bottom and you will see two steel levers which you need to move. You then lift out the brush roll: pay attention while you do this to see how it comes out, as that knowledge will help when it is time to put it back in. You then place the new belt over the brush roll and then replace the brush roll. On my Gsix, one of the plastic brush roll ends is larger than the other. The roll will only fit in one way, so flip it around if you can't get it to smoothly slide in place. Maybe it is the same on your G4.