Depends on the amp. Many 4-input amps give you the opportunity to "jump" the inputs. By that, we typically mean plugging an instrument into one input from Channel A and then running a short cord from the second "A" input to an input on Channel B. You can mix and match as to which path you actually use. The end result is usually a nominal increase in the punch and volume, as both volume controls are now interacting. Again depending on the amp (two sets of tone conrols), the tone controls may interact as well. I do this with my SVT, and the trick is also common on four-input Marshalls. I've tried it with a Fender Super Reverb, and it didn't work.
