You can run 2 full-range speakers off each head unit channel if you wire them in series; however it would cut the power output of the head unit, and the reduced power would be split between both speakers.
You can run two 4-ohm speakers in parallel if you use crossovers so they're playing different frequencies. For example, you can run a 4-ohm mid and a 4-ohm tweeter with a 2-way crossover; that keeps the average load at 4 ohms. It's usually also not a problem running a full range speaker in parallel with a tweeter, as long as the tweeter has a crossover filter.