Bb 7sus4 (Bb7sus4)
Also written as A# 7sus4 (A# and Bb are the same key).
Notes in Bb 7sus4
Inversions
Same notes as Bb 7sus4, with a different chord tone as the lowest note. Useful for smoother movement between chords instead of jumping around the keyboard.
Root position
A# in the bass
1st inversion
D# in the bass (Bb7sus4/D#)
2nd inversion
E# in the bass (Bb7sus4/E#)
3rd inversion
G# in the bass (Bb7sus4/G#)
Open voicings
A close-position chord like Bb 7sus4 can sound cluttered in the middle of the keyboard. Jazz players open it up by dropping one note down an octave, spreading the same notes wider without changing the chord.
Drop 2
E# on the bottom
Drop 3
D# on the bottom
Bb 7sus4 is built from A#, D#, E# and G#.
A dominant seventh chord with the third swapped out for a perfect fourth, combining the open sound of a sus4 with the pull of a dominant seventh. Gospel and blues piano lean on it heavily, often as a stepping stone right before the full dominant seventh resolves.