Ronbo is correct. The speaker IS the load. The amplifier is made to work most efficiently when operating with 2 ohm loads (speakers). When using 4 ohm speakers, it would take twice as much voltage to push the same amount of current through the speakers compared to the 2 ohm speakers.
I don't know the exact voltage or current the amplifiers are running at, but for example, if an amplifier operated at 12 volts with a 2 ohm speaker, it would put out 6 ampheres (or 72 watts).
Using the same amplifier, operating at 12 volts with a 4 ohm speaker, it would put out 3 ampheres (or 36 watts).