If it is the 4 channel amplifier found here
http://www.pyleaudio.com/manuals/PLMRA220-420.pdf then you want to purchase either a pair of 4 ohm single coil subs OR a pair of 2-ohm dual voice coil subs and hook the coils together in series to give you a 4 ohm load. It says your amp is stable down to 4 ohms when bridging the channels together(which is what you want to be doing, like the second picture down on page 10).
So in closing, go with either 4 ohm single coils, or 2 ohm dual coils.
Nic