IIRC the 4M+ has four 72 pin SIMM slots one of which is used by the Postscript SIMM. You can install standard 72 pin SIMMs into the other slots to increase the printer's memory. HP always claimed it needed special HP memory but that was a scam. In one case I even had an HP printer which wouldn't work with HP memory but worked fine with generic 72 pin SIMMs.

I think there are some ID resistors on unused (PID) pins but most SIMMs seem to get those right.