to clarify you intend to bridge a 4ch amp to 2ch and feed a single sub ?
If the sub is dual voice coil then fine as long as you DM the levels , if its a single coil then no you are looking at probs.
You will really struggle to match the levels correctly unless you have acess to a true rms volt meter.
The easiest and most benficial method would be to run the 2 subs isobaric style !!!! this way you get WAY WAY more bottom end reinforcement and dont need to increase your box size. It is by far the best method for running lower powered subs to gain maximum effect.
PM mr if you want more info on going iso with your subs.