Originally Posted by
Indefinite_Cure
Hmm, not to be "that guy" but...the fourth cannot really be "flatted" in a mixolydian mode. A fourth can either be diminished, perfect or augmented. A diminished fourth is equivalent to a major third. In a mixolydian mode, the fourth degree is a perfect fourth from the fundamental, which means, if, still in a mixolydian mode, the fourth degree was to be flatted, it would essentially result in playing the 3rd degree of the scale twice and then jumping to the 5th.
In other words, here is an example:
Mixolydian C scale:
C-D-E-F-G-A-Bflat-C
Moxilydian C scale with flatted fourth:
C-D-E-Fflat-G-A-Bflat-C
F flat = E
In a lydian mode however, the fourth is augmented, which means flatting it would result in changing your lydian mode to a ionian mode (basically a major scale).
C lydian:
C-D-E-F#-G-A-B-C
C lydian with flatted fourth:
C-D-E-F-G-A-B-C
/end musical theory rant.