I would like to replace the 8 ohm speaker in my Rumble 100 V3, with a 4 ohm speaker. Can anyone tell me if I will get an increase in output/volume?
Pop into the Rumble club and pose your question. The member @Linnin and others have much experience in such issues. Fender Rumble Club
That combo amp does not have an external speaker out, nor does Fender advertise that the internal amplifier is stable at 4 ohms. My assumption is that you would get a brief, slightly louder volume before the amplifier goes into protection mode.