that's not quite how it works.
the amplifier provides a voltage.
the lower resistance of the 4 ohm driver allows more current to flow through it.
this draws more power out of your amplifier and flows more power through the speaker.
if both your amplifier and speaker can take the increase in current, then you will get more spl out of your speaker.
many pro-audio amplifiers do not like to be bridged into a load less than 4ohms, so that is why you won't see many subwoofers with a less than 4 ohms rating that are meant for home or pro-use.
also, two 8 ohm drivers in parallel give a 4 ohm load, so if you want to parallel two drivers, the 8 ohm option may be the better choice.