Epson AcuLaser C4000 Reference Manual page 240

Caution:
c
Do not force the memory into the slot.
Be sure to insert the memory facing the correct way.
Do not remove any modules from the circuit board.
Otherwise, the printer will not work.
Make sure that one module (more than 64 MB) is
installed in the S0 slot.
7. Slide the circuit board tray into the printer.
228 Using Options