why mono?

Because it is not producing left and right anymore.

You can bridge channel 1 and 2 to power a component system in left door or kick panel, then bridge channel 3 and 4 to power component system on right side, my 5 channel amplifier outputs 75 watts per channel (1,2,3 and 4) at 4 ohm so there was no need to bridge any of the channels but if you have a low power 4 or 5 channel amplifier and want to drive a high power component set then bridging 1/2 & 3/4 is ideal..

To simplify things, you are combining 4 low power channels and creating 2 high power channels..

OP, Keep in mind channels 1/2 and 3/4 are separate from the 5th channel (as far as all the 5 channel amps ive seen anyway). Bridging the first four channels into 2 channels won't change anything with your sub