Old amplifier & impedance diffrences.

Using lower impedance speakers puts a higher load on the amp *if* the speakers have the same efficiency (if you use very efficient 4 ohm speakers vs very inefficient 8 ohm speakers you can get more volume at less load with the 4 ohm speakers).

The speaker impedance given by the manufacturer can be a bit misleading though since there are different norms iirc, some 8 ohm speakers can go down to 2 ohm at certain frequencies and other 4 ohm speakers are all the way stable.

You should check the temp of the amp if it stays low to moderate at your listening volume you're fine.

There should be audible distortions in the sound at higher volume if the speakers have too low impedancy values.

mh