You can run whatever you like as long as your amp can handle it. Generally the speakers with the lower ohm coils will be louder due to less resistance, but alot of other factors come into play as well.
For anyone who thinks this can't be done, please explain why without guessing. If your 4 channel amp is rated for 2 Ohm stereo operation there should be no reason you can't run one pair of 4 Ohm speakers on half the amp and 2 Ohm speakers on the other half.