a 4 channel amp or a 4/50w radio means you have 4 sets of speakers outputs, usually a left front and a right front and a left rear and a right rear.
which means you should be able to wire up all 4 of your speakers.
get a dual channel (left and right) crossover and wire it to the rear left and right channel outputs near the back of the car. from the crossover, wire the bass outputs from both the left and right channels to the subwoofer, all the other outputs from the crossover (mid, tweeter, etc.) wire the left sides together and run to the left speaker, wire the right sides together to the right speaker.