Mike,
just to clarify... there is no reason that you can't load an amp with one side on subs and the other full range - as far as the amp is concerned. A modern stereo amp is basically two channels, that can be run independently (stereo) or together (bridged).
It should run with a 4-ohm load on one channel, with an 8-ohm load on the other. No problem.
Chuck's point was that you wouldn't want to run a sub and a full range speaker together (no matter how you power them). This leads to both the full range and sub duplicating lower frequencies which will make your sound muddy. If you have a crossover that splits the low freq. sound to the sub and everything else to the full range, then by all means power each off one channel of the same amp.
However, you will probably need the same configuration on both sides of the stage - whether running mono or stereo. If mono, you could get away with a [4 or 8 ohm] sub on one channel then run two 8-ohm full range speakers off the other. Just make sure you amps power output will run both of them and provide 1.8->2.25 times the RMS wattage of both speakers - since you'd be powering them off the same singel channel. i.e if your speakers are 200w RMS at 8-ohms, then 200*2 = 400w at 4-ohms. So your amp should be able to handle between 720 and 900w at 4 ohms per channel.
If stereo you will need a second amp, so you would [normally] run tops off one amp and the sub (or subs) off a second.
Steve