I too bought an MP4 years ago, because "I couldn't resist." I've gotten good use from it. I've attached some photos of what came with the camera and accessories I've picked up along the way.
PHOTO A: The first shows the sliding base that came with the MP4.
PHOTO B: The second shows attachments one can use to shoot 4x5 sheet film with sheet film holders. The "C" on the right holds in the film. The ground glass is on the left. Notice the pins that extend to each side of each device. Spring clips on the base hold the accessories in place by clamping on these pins.
PHOTO C: Sliding base with sheet film attachments installed.
PHOTO D: Polaroid film holder. Has the pins on each side. Could use this in place of sheet film holder (on right) in PHOTO C. (So, shoot with Polaroid instead of sheet film.)
PHOTO E: Theoretically, one could shoot with medium format film. Both this particular ground glass and the film holder just lay flat on the sliding base. There are no pins on the film holder; so, I'd play around with holding the film holder in place with a small, weighted sandbag or something.
They also made an 8x10 film setup for this device, but it's been years since I've seen one on EBay. I have the short column version of the MP4, so I don't know if that would be practical.