Is any 4 channel amp capable of running in 3 channel operation? As in, 2 channels for the highs and bridge the other 2 for a sub? Or is this amp specific?
There is no reason you couldn't bridge two of the channels (as 94Mazda626 said), as long as it is bridgable. If it is, this will be noted on the speaker connection terninals of the amp. You also want to make sure that the amp is stable to whatever impedence the sub will be at, once it is bridged.