2 channel amps have been as sub amps since the beginning of car audio. In fact, they were most commonly used as sub amps before class D mono amps came to dominance in the late 90's. As long as it has a LPF or "low pass filter", you are fine (99.9 percent of 2 channel amps have a LPF).
So, yes you can. You "bridge" the 2 channels into 1, mono channel, as shown in the owners manual.
A four channel amp will work, but its a terrible idea for a sub amp, they are primarily used for interior speakers. You would only be able to use 2 of the 4 channels for a single sub.
Underpowering doesnt damage speakers like some newbs say. Just make sure the gain is set correctly... many people buy amps too small, and are not happy with the loudness, so they jack the gain all the way up and damage the sub.
