“Fourth chords” are chords built as a “stack of fourths,” rather than as a “stack of thirds.”An example of a “stack of fourths” would be: D, G, C, and F, where D is the lowest pitch, and the rest make up a series of fourths above that.
The greatest thing about these stacks is that any given stack can be superimposed above multiple roots, to create a variety of voicings for various chord types.
Using the stack mentioned above as an example:
A “Dmin7” chord using the stack D, G, C, and F, results in a nice open-sounding voicing, with an added 11th (the “G” note is the 11th).
D, G, C and F also sounds great over a B-flat root, creating a “Bb69” sound! That is, a B-flat major chord, with an added 6th and 9th. (G is the 6th, and C is the 9th).
And so on…my video here explains this in depth. (Check back soon for Part Two, with more insights on this.)