At the same input voltage a 4-ohm driver will draw 3db more power than an 8-ohm driver. If both drivers have the same SPL at a given watt then, yes, the 4-ohm is louder. Both have the same efficiency.
But now you have two different sensitivities because with the 4-ohm driver you get 1 Watt from 2 Volts whereas you get 1 Watt from 2.83 Volts from the 8-ohm driver.
But now you have two different sensitivities because with the 4-ohm driver you get 1 Watt from 2 Volts whereas you get 1 Watt from 2.83 Volts from the 8-ohm driver.