"Music for 18 Musicians" by Steve Reich is probably one of the masterworks of the second half of the 20th century.
Any vaguely disco-adjacent band will have more than 4 people on stage because there will be at least keyboards and horns in addition to drums, bass, guitar and vocals. Even a band that simply adds an additional person player percussion to a typical 4 piece exceeds your limit yet can wonderfully enhance the music.
If you haven't seen any of those bands, then that's your loss, but provides no reason to try to generalize about the right size for a live band.
Any vaguely disco-adjacent band will have more than 4 people on stage because there will be at least keyboards and horns in addition to drums, bass, guitar and vocals. Even a band that simply adds an additional person player percussion to a typical 4 piece exceeds your limit yet can wonderfully enhance the music.
If you haven't seen any of those bands, then that's your loss, but provides no reason to try to generalize about the right size for a live band.