Firstly, when using a 4 channel amp as a 2 channel for left and right, set both channels EXACTLY the same. If you use just 2 channels of the 4 channel amp, then as far as I know you are only using half the power. But if you bridge channels 1 & 2, and bridge 3 & 4, you'll be using full power. Around the terminals it should indicate which positive and which negative to bridge. If not, then I think it is the 2 furthest apart terminals, negative and positive of course.
So basically, bridge 1&2, and bridge 3&4.
P.S. Karl, I have been thinking of this, and I'll get onto Steven about it.