You can do it, but your sub would try to project the entire decible range.

on the amp you have: (all set a full mode, not just mono)
- front right
+ front right
- front left
+ front left

- rear right
+ rear right
- rear left
+ rear left

Wire the front speakers normally, then bridge the amp to the sub. That should get you power, but since the amp is on a stereo mode for the front speakers, the sub might not get as low as you'd like it.

*disclaimer - this is going off of 5 year old amp technology, i need a new one and current models may be completely different from what i have. h: