Seems to me it would have made a lot more sense to go with an 8 ohm tweeter and a 4ohm mid, if was going to run a 4 channel. Id go with a large 2 channel, then a smaller one for the tweeter.
Id consider the rl-p 18, you have about 6.2 cubes before displacement and port. The SS website recomends 5 cubes at 27 hz, plus its got one of the sexiest rear ends in car audio. Also the lower power demands will allow less stress on your electrical system.