Go here for product specs for the LJ 4M & 4MP. Basically, it says that it comes with 2MB RAM standard and towards to bottom of the page it says it has two SIMMs slot for additional RAM. It also explains "The optional memory required for the HP LaserJet printers is directly influenced by the complexity of the graphics and soft fonts being used." It then talks about the type of SIMMs required: "SIMM specifications for the HP LaserJet 4p/4mp printers are 72-pin, 32-bit, and 80 ns. Presence Detect must also be available on the SIMM. (The HP LaserJet 4p/4mp printers can use 36-bit memory from the HP LaserJet 4 printer.)"
The other factors limiting print speed is that the parallel port can only transfer so much data at a time...complex pictures, graphs, spreadsheets, etc. do take quite a while to download.
EDIT: I just checked the Service Manual for this...I have it here...and it looks like the the 4MP comes with 6MB standard and has two slots for additional RAM. You can use two 8MB SIMMs to max out at 22MB.
