is the bed size the same on the 4wd reg cab as on the 1999 4wd extended cab and could i use same cap

The bed could very well be the same size for a regular cab vs an extended cab, but I would not assume this is the case. Measure your bed and then compare that to the end caps you're interested in purchasing.

As "dvigano" said measure the beds and the cap but also check these measurements; #1 - Front "bed to frame bolt" location to the front of bed sheet metal. #2 - All bolt locations by layout "front to rear" and "side to side". #3 - The fuel filler neck locations.