A second amp may be all it takes to sort you out, that gets you two 4 ohm cabs with the 4pro bridged into one. Although if I wanted a second amp to complement the 4pro if pick something with a grindy power section like a 400rb, or a hartke MOSFET 3500. The sunn wouldn't be my first choice betting an ultra linear design (IIRC).
What do you mean by ultra linear design?