With a 4 channel amp, use the front channels, L&R, for the 5 1/4" component set, and bridge the rear channels L&R, following the manufacturers instructions, for the sub.
A good 4 channel amp with a rating of 40 watts/chan RMS into 4 ohms will put out about 160 watts, X2, when 2 channels are bridged into 4 ohms.
The bridged channels see half the impedance that they are connected to. A 4 ohm load is seen as 2 so don't put too low an impedance on the bridged channels. Again follow the manufacturers reccomendations.