Hi Nik, adding an 8 ohm cab to your 4 ohm cab would need 2.67 ohms from your amps, so that's good. The power would be split 66% to the 15 and 33% to the new cab. I'd check with Vanderkley, they could advise if their 210's are designed to work with their 15's. I believe their 112's and 210's were designed that way.