4 ohm speaker in place of an 8 ohm

[ppppenguin]

Yes. And the value isn't critical. Any resistor or combination of resistors that add up to between 3 and 5 ohms will be fine. I don't know the power rating of the amp but I'd rate the resistor fairly generously. 4R7 20W should be OK. Or a pair of 2R2 10W in series. Or pair of 10R 10W in parallel. I'd be raiding my junk box.