4/3
| Interval information |
reduced,
reduced subharmonic
[sound info]
4/3 is the frequency ratio of the just perfect fourth. Its inversion is the perfect fifth, 3/2. In the florid organum of Medieval music, 4/3 was reliably considered a consonance, and indeed was frequently emphasized. Once major thirds with a tuning approximating 5/4 began to be treated as consonances, however, the perception of 4/3 was altered to where it was at times considered a dissonance.
The 4/3 interval is easily heavily discussed outside of xenharmony, as the corresponding Wikipedia article makes abundantly clear. In fact, some of the usages discussed there have gone on to inspire other music theories in xenharmonic contexts, and indeed continue to inform certain ideas about tetrachords.