Most modern amps will drive 4 Ohm loads without too much complaining. It depends on how difficult a load the speakers present. A simple 2 driver speaker with minimal crossover components would be no problem, while a 3 or 4 way box with extensive crossover and low efficiency would be more of a concern.
Small Sub Sat systems will also be fine. Many of these are 4 Ohm to get more volume out of small amps, but are easy to drive.