Chord sequence
Gmaj7 B♭7 Am7 A♭7
In every key
Four-note rootless voicings in their two forms, with the 3rd or the 7th on the bottom, played over the root.
Heard in G major
All the chords
The I-VI-ii-V with tritone substitutes for its dominants, bIII7 for VI7 and bII7 for V7, so the bass falls by half steps from bIII back to I.
Over Gmaj7 · Imaj7
The major scale and its modes are the most commonly used scales in western music.
Over Bb7 · subV7/ii
Mode 4 of Melodic Minor
Over Am7 · ii7
Mode 2 of Major
Over Ab7 · subV7