Well, a buddy of mine just screwed up a solid state amp due to some ohm mismatches, so I'm being more careful now. Unless that 4x12 is 4ohm (which it is not per your post), don't have the amp plugged into the internal speakers at all. Sooooo - just plug into the 8ohm input on the 4x12 and unplug the internal speakers. The outs are 4ohm each, so going to an 8 or 16 ohm cab ONLY (no internal) is not a problem, you might experience some volume drop, but not much. I'm going to look something up and I'll post more maybe in a few minutes. BTW, I just looked at the back of mine to verify the 4ohm outs.