The lower the speaker impedance, the more current (amperage) your amp or receiver will need to provide, but most separate power amps have more than a sufficient power supply and power amp sections for driving 4 ohm loads with ease. In fact many amps will perform better with a 4 ohm load as opposed to 8 ohms if their design is robust enough to supply the extra current. A quick web search will bring up more information on this subject.