As others have noted, the relative sensitivity of the speakers is probably more a factor than the impedance.
Although an ideal amplifier will deliver twice the power into 4 ohms compared to 8, a 3dB increase, real world amplifiers don't behave that well and will start to current limit into a 4 ohm load at a lower output voltage compared to 8.
Swapping an otherwise identical 4 for an 8 might just be audible but new strings or a drunk guitarist far more a factor.
Wait - we are talking about a solid state amp? If it's valves, I'll get my coat...
Edit: sorry, Ossyrocks was more concise.