The best solution would be to assign one channel of the amplifier to each speaker. This is not a big concern, as there are proper methods that a qualified installer can use to have a 2-channel amplifier driving a 4-speaker system. For instance, a 50 W x 2 or 50 W x 4 amplifier could be good configured to drive a 2 or 4-speaker systems.
If you plan to install a subwoofer as well, a 5-channel amplifier, with a 5th channel dedicated to the subwoofer, is surely the quickest and easiest solution. Since the subwoofer section of the system generally requires higher power than the high frequency ranges, the fifth channel usually delivers more power (watts) than the front and rear channels. For instance, a 5-channel amplifier could have the following power configuration: 50 W x 4 + 100 W x 1 channel (at 4 Ω).
Most amplifiers today can have two channels “bridged” together. When “bridged”, a pair of amplifier channels is combined to create one single channel with higher power, thus making the installation more flexible. With this set-up, a 50 W x 4 amplifier could be configured as a 3 or 2 channel amplifier with the following power output: 50 W x 2 + 200 W x 1 or 200 W x 2. Because of the higher power, the “bridged” output is ideal to drive the subwoofer.
REMARK: Some amplifiers feature a remote level control which lets you comfortably control the volume of the subwoofer from the driver's position. Since the intensity of the bass output can vary drastically from one musical genre to another, the remote subwoofer volume control feature is very useful.