My problem is that I want to play several programs in the MPC simultaneously from an external sequencer.  With most MIDI devices you simply load a multi and assign each program its own MIDI channel.  The MPC doesn't work this way, and as far as I can tell you can only play multiple programs at once if you are using the internal sequencer (by assigning each track one of the four Drum kits). Any work arounds???  How do I play all 4 Drum kits from an external sequencer if they are all using the same MIDI channel?

Here's how:

I assume you already have several drum programs loaded into the machine. First, of course you have to map your 4 available DRUM programs that will be available to the sequence, i.e.  DRUM1 -> program1, DRUM2 -> program6,
DRUM3 -> program9, etc. (your choice).  Then assign each of the different DRUM1-4 to a different track.  Easy so far, since you are probably doing this already.   Go on the main screen to the lower left of the screen where there is either 'M' or 'S'.  Rotate the data wheel to change this to 'M' then click on OPEN WINDOW.   Here you will the multi-track recording (playback) setup.  This is where you can assign each of the incoming midi channels to a particular track.  On the left you have 1-01 through 2-16 (there's 1-Ex and 2-Ex for recording sysex data on their own track too). Pick each input and channel to set up and assign these to their appropriate track.   Here's and example:
1-01 > track1 > off  (where track1 is DRUM1)
1-02 > track2 > off  (where track2 is DRUM2)
1-03 > track3 > off  (where track3 is DRUM4)
'off' is the output channel for that track, which if you are triggering internal sounds only then you won't need midi going out of the machine.

Go back to MAIN SCREEN. on your external sequencer go to the track that outputs on midi channel 1(or whatever assigned), you will hear the program assigned to that track. Go to next track for other midi channel and check there also, you will hear the other drum program.

Voila, 4 part multitimbral drum sampler module.