To configure two 4-ohm speakers to look like an 8 ohm load you need to put them into series, one speaker after another. If you are talking instrument speakers, often they will have different jacks on the back that allow you to do this. If not, what you need to do is run one of the two wires out of the amp to one connector on the 1st speaker, then run a wire from the other connector on the 1st speaker to the 2nd speaker. finally run a wire from the remaining connector on the 2nd speaker back to the amp. If you have to do it this way, watch the + and - on the speakers, this will effect 'Phasing' if you get them backwards. This means: Amp to + on 1st speaker, - on 1st speaker to + on 2nd speaker, - on 2nd speaker back to the amp. And do note that Instrument speaker cables and jacks have the two 'wires' in the one jack. As to 'common ground', not totaly sure what you are asking about. By definition, in a series configuration only the 1st speaker will be 'at ground', the 2nd speaker will be somewhat higher. But this does not matter unless you have some other odd thing going on with the speakers creating a ground loop. Like trying to tie one terminal on a speaker to the supporting metal framework, or if you have some odd type of 'active' speakers, ala with built in amps. 'Common ground' (And things like Ground Loops, and introduced humming) really only come into play on the instrument side of things. By the time you get through the Amp and into the speaker, these do not really matter. Hope this has helped