The ACX-300.4 has the potential to output 150W RMS for 4 Ohm speakers when bridging two outputs together. To do this on the channels 1-2 output, connect the positive lead from output 1 (White), and the negative lead from output 2 (Gray/Black) to your loudspeaker. For channels 3-4, connect the positive lead from output 3 (Green), and the negative lead from output 4 (Purple/Black). Altogether, it should look like the image below:
The mono loudspeaker will receive the power from each channel, and both channel inputs must be fed an input signal. The minimum impedance is 4 Ohms.