If the amp was designed to be bridge it would have it printed on the unit by where the terminals are labeled. There is no (well, no good way) to bridge the amp you have from 4 into 1 channel. Best bet IMO is to pick up a decent 2 channel bridgeable amp or a mono amp if you don't plan to run another sub at some point.
FWIW, i'm running an Alpine MRP-M350 that I scored on ebay for like $60 IIRC. It powers my rear deck sub which is, mind you very efficient but would have more than enough power to run a single sub. Plus, consider the fact that you ideally want an amp that you can adjust the crossover point (I'm assuming you don't plan to run an external equalizer or anything like that), running a crossover point much higher than 90-100hz typically ruins imaging. The human ears can typically tell where a sound is coming from at any frequency higher than something like 100hz IIRC, so a crossover for your subs at or below that point helps improve imaging and helps the sub "blend" in.