Paladins and Templars are likely no longer around, at least not the ones we’ve seen thus far. Paladins were mostly absorbed within Westmarch Knights and thus focused more on physical threats. Templar Order was destroyed (save for some remnant members who weren’t there) by Kormac and the Nephalem. So if they return, it’ll be a new order that’s using the name imo.
That leaves Monks and Crusaders considering we don’t know of their fates by the time Malthael was defeated. However, due to the Zakarum religion losing even more influence by the time of Diablo 4, the Crusaders may be struggling even more than before, especially when one considers how they recruit new members.
That said, there could be easily be other faction that could do it in terms of lore. However, whether they would qualify as a playable class is another matter altogether.
Same.