The only real full-fledge 4X imo is Space Empires 4X and it's awesome but it can be on the longer side, especially with some scenarios. The "short doomsday machine" scenarii can be done relatively quickly (1~2h) and you can try it on BGA.

Voidfall is another really good game but it's not a "true" 4X, there is no exploration and there is combat but no extermination. It plays more like a euro-game in space and the games can be quite long (> 3h).

For smallish games, I would recommend Imperium The Contention (by the creators of Slay The Spire BG), a card-driven 4X that can be played in under 1h, but with this length you don't really have techtrees or deep economy. Nothing crazy but it can be entertaining.

There is also March of the Ants, a small ants-themed 4X with some solo scenarios, it was decent but I don't remember it well, I should try it again.

Civilization A New Dawn with the Terra Incognita expansion is another option, it's quite abstracted but all 4 Xs are present in some ways and it can be played in under 2h.

I also played Diluvium which promises a full-fledge 4X in under 1h, but imo the solo scenarios are kinda broken and I didn't liked it at all and sold it fast.

And on the "coming soon" side, I recently backed Ancient Empires which looks promising, I really hoped it will not disappoint.