More power does equal more loudness, but not as much as you might think. It takes twice the power to get a 3 dB increase in loudness. 100 watts will produce 3 dB more loudness than 50 watts (10*log(100/50) = +3.01 dB). 3 dB is noticeably louder but not that much.
But to compare a 4 ohm speaker to an 8 ohm speaker, you have to compare apples to apples. - they have to have the same sensitivity. If they have the same sensitivity, then they will produce the same loudness but the 4 ohm will draw more current and the watts will be higher. V=IR but the V is constant as an amp/receiver has a fixed rail voltage, so as the R goes down from 8 to 4, the I must increase to maintain the same relationship. [Simplifying because the R actually varies with frequency].
