They are different engine families. The PW2000 is in the 40,000 lb thrust class, while the PW4000 on the 744 or 767/MD11/A300/A310 (94 inch version) is in the 60,000 lb thrust class. Which is to say that the PW4000 is larger and more powerful than the 2000. There are many differences and some similarities in the 2 engines. But they are definitly different engines. In fact there are way too many differences to list, however to mention just two big ones, I would say gearbox location, and FADEC system. Just to make things interesting, the 100 inch (A330) and 112 inch (777) PW4000 are different also, even though Pratt's marketing department would never admit that.
As for why different engines for 2 apparently similarly sized aircraft, check the max takeoff weight.