(above ratings are per sub, eg you can run 2 dual 2 ohm subs off a 4 channel amp
dual 4 is for competitions and can get expensive nice subs available for $300 in dual 2's.
This has no relevance on what your stock system is, however, to run a sub in the first place check to make sure that your deck has sub out, otherwise u have to use a line converter off your back speakers, low voltage, not best idea.