I just got an email from Electro-Harmonix, and here's what they had to say: Hi Ben,
Unfortunately the unit does not work with a 4 ohm cabinet.
We do not have a fix for this at this moment so if the unit does not work for you, you may want to return it.
Which kind of conflicts with their product description, Electro-Harmonix is pleased to announce the new 22 Caliber Power Amp, a versatile 22-watt guitar head that fits in the palm of your hand. Just plug it into any 4-, 8- or 16-ohm speaker, and you'll get a big, sweet musical sound. NOT NECESSARILY!!!!