With both 8 and 4 ohm speaker connectors you only need to hook one of them back to the PCB (this is for the global negative feedback loop). Usually this would be the 8ohm tap, wich gives slightly more feedback. The 4ohm just goes to the 4ohm tap of the output transformer.
FWIW, i wired my OPT to use the 4ohm tap... always. So i connect 8 ohm speakers to the 4ohm tap. This sounds best to me, but is likely speaker dependant. (and I've disconnected NFB loop also)
try for yourself, as usual
enjoy,