Well since the center channel and subwoofer frequency ranges do not cross at all (Each set of amps/speakers would filter out the others freq range) It's apparently being used for both. So if you have a center channel AND subwoofer output you need to put a Y cable into the orange jack and run one to the center and the other to sub.
It LOOKS like the way the sound is setup you don;t get any sub if you don;t have a center channel
but sometimes the manuals can be a bit terse and not fully explaing things. Give 2.1 a try, it'll either work or it wont. But given that 4 ch has no sub listed there may not actually be any 2.1 either. I'd try it anyway, you never know.