A-Best: it depends on how much power you need. most amps will have no problem with a 4 ohm load, but the catch is they will not produce their "rated" output power. For example, a 40 watt rated amp might only develop 30 watts, or 15 watts per speaker instead of the 20 watts per speaker. for listening in a room this is plenty adequate. For dance halls and auditoriums, speaker and power management requires better matching for optimum performance.
