I have a rockford fosgate punch 150 and it will power a small 5 1/4" speaker, but doesn't seem to push my 15" subwoofer. I am reading 4ohms across the sub so I'm assuming that it is fine and that there is something wrong with my amp.

Amp is way too small to power a 15 inch sub. But you should get some sound out of it, not silence. You can use a 9 volt battery to test your sub, if the sub pops when the 9 volt is placed across it's terminals then it's ok. If it does not pop then your sub may be bad, or the coil may be frozen. (It can still read 4 ohms if frozen). If your amp powers smaller speakers fine, you know it's not the amp. If the sub tests ok, then verify your connecting it properly to the amp's correct bridging terminals.