I don't have a Sunn in front of me, but I think the left jack is the primary so if it's set up for 8 and 4 ohms, the right jack should already be set up for running a single 4 ohm load. Also, there
are threads describing the jack wiring in detail, but I sure couldn't find them with a quick search. Maybe you can have better luck searching than I did, I know there is specific jack wiring and switching info
somewhere...
I think it would be a shame to cut another "un stock" hole in that pretty amp.