If you've got a voltmeter check between pins 7 and 8 on either joystick port. There should be 5v there to power the electronics in the mouse. The buttons don't need it since they are just switches. If the 5v isn't there you'll get no mouse movement but still have the buttons. I've seen a lot of 2000's fail thia way. On the original 2000s a short here would melt a trace on the motherboard. The updated it to include a 'pico fuse' to protect the circuit. It was right by the joystick ports on a 2000, but I don't know about the 4000. My schematics for the 4000 shows a fuse F175 in the 5v user line (all external ports with 5v). I don't have a board layout, so I can't tell you where that is.
Good luck
