While it wasn't off the table, DLC for Streets of Rage 4 wasn't necessarily confirmed either until now. We finally got confirmation that there will be future DLC for Streets of Rage 4. In the tweet there's a pretty obvious hint of Estel (a highly requested playable character) in Dotemu's tweet:
Happy to have literally any excuse to play more SOR4
Happy to have literally any excuse to play more SOR4