Yep. With a 4x12" you can have the same SPL you get with a 1x12" whose sensitivity is 6 dB higher.
Feel free to turn the trimmer clockwise as long as you not perceive any distortion. I'd advise you to use a sinusoidal signal (it's easier to perceive new harmonics), and you'll have to do it with the 4x12" and at rehearsal volume (or you won't be able to hear a contingent distortion.
HTH