IDK, could be a worn/defective port. But as far as it being slow...if I'm not mistaking, the 4k is the only mpc with usb that uses the 1.1 usb protocol. Keep in mind that the 4k hit the market when usb 1.1 was still in play. 2.0 had just hit the market in the early 2000's but nobody had started using it except for the major computer manufacturers. Whenever the next mpc came about, it was sporting the new 2.0 protocol (which operates much faster than 1.1). I think everybody that owns a 4k and uses it to load/save material via usb has to deal with this problem (it being slow).
MPC 4000
SSL Fusion
Roland JV1080
Ensoniq ASR10
Pioneer XDJ RX2
Korg Triton Rack
Kurzweil K2500RS
MiniMoog Voyager
Yamaha Montage 7
Yamaha Motif ES Rack
UA Apollo X8 Interface
Tascam DM4800/DA3000