MPC is a universal tool, there loads what you can do with it, even controlling discolights and vj software, dimmerpack dmx the lot...It just as how silly you want it to handle, I use it as tape player/recorder to midi gear...I sit behind a masterboard somewhere or a clavinova, hit the metronome and start prssing record... Hook it up to any vst wrapper/loader and hotwire the shit to vst's and screw the software sampler, you pull that one on a emu card and you got youself an mpc with SP1200 sound. Hell, I use it as mpd16 hehe. Or complete produce complete tracks on it with edited samples from pc, livesets with other midigear due to 32chnls(techno/elektro, I got rack for that shit). I believe the key to this machine is simplicity, no fancy stuff just midi and a S2000 sampler, nothing special actually besides good pades and a tight-as-fuck sequencer.
MV8000 is like the ultra advanced MPC, in fact, it a competitor to the 4K. I dont know which one is better since I dont have a purpose for it, I'd just have more s/w fx and dynamics which I'd rather not use, multitracking options to disk, VGA out and Mouse, good sample editing/slicing, midi writing capabilities, midi functions and mixer. They both probably have it all, but if you're using a pc you have 2 daw's ?
That's the difference between mpc1/2k and the topmodels like MV8000/MPC4k, one can be a DAW or core in your daw, the other is a DAW (for the noobs, digital audio workstation).