As the name implies, the Four chord is a sort of sub commander in terms of tonal gravity within the major tonality. It can suspend the motion of tonal gravity and create the illusion of holding it in place without the loss of artistic direction and forward motion. How? Well, the Four chord can play the important role of providing a secondary stable resting point within the major tonality, with similar aural and theoretical properties to that of the tonic / One chord. In many ways, One and Four can be the exact same chord. Really? Yep. Hear the subtle sense of temporary rest provided by the Four chord in 2nd bar of the following idea. Example 4.
| I | IV | iii ii | I |
In the more basic forms of American music, folk, rock, pop etc., the harmonic motion of One to Four is probably the most popular harmonic motion for writing songs. Oftentimes the melodic idea used over the tonic is simply transposed over Four. Here is a tonic / subdominant "carnival" vamp. Example 4a.
| I | IV | I | IV |
Many great tunes use the above vamp to great effect, dancers love it too. Just a simple, rocking motion between One and Four, does it get any simpler? This last idea places the One, Two, Three and Four chords into a 4 bar gospel vamp. Example 4b.
| I ii iii IV | IV iii ii I | I ii iii IV | IV iii ii I |
The pop / gospel tune "Lean On Me" uses a similar motion. Do you know that tune? The chords go by pretty quick, try to get these changes under your fingers at the piano. All ready got it? Try running this lick through the other 11 major keys. Oh, what does the term Lydian mean to you?