It's a 4 ohm speaker to get the most power out of the small solid state amp. An 8 Ohm speaker will just about halve the output power the amp produces, down to 5W from 10W, but in reality the volume drop will be minimal, about 3dB in the SPL produced, which isn't a lot. However, a good replacement speaker may well be more efficient than the original, and produce more dB SPL for the same power input.

But the reality is it's a very basic practice amp, so I wouldn't spend too much on a replacement speaker. You wouldn't have to spend a lot more to get a much better sounding basic modelling amp.