You are meant to match the ohm rating (impedance) of a new speaker with that of the existing. Common values are 4 ohm and 8 ohm. You should not use 4 ohm speakers in the place of 8 ohm, but it should be OK to put 8 ohm in place of 4 ohm, you just won't get as much volume.

If you want to make the most of the speakers you could put an additional amp in the pinball machine.