I would not rule out Diablo 4. After they turned Diablo 3 basically into merely a prettier looking Diablo 2 they might as well redo Diablo 2 again plus loot boxes.
People don't even want anything different. Seems everyone is perfectly happy with Diablo 2 rehashes. The dumber and simpler, the better. Just click, click, click and smoke weed.
So why waste time and risk to miss billion$ in case loot boxes get the gambling ban hammer or some shit like that? Even Blizztard might be able to put the pedal to the medal in such an environment as we have right now and crank out Diablo 4 sooner than expected with a nice GaaS business model... base game for $69.99 and then one expansion after another for $49.99/piece plus new characters for $29.99/dude and loot boxes galore.