I would remaster 2 and 4 since I think a proper 2 remaster that's anything other than "in the spirit of 2" would be too rudimentary.
When it comes to remasters, there are these outsourcing firms that have don't have the art direction skills to do it right. See: Advanced Wars for Switch. I'd take Civ 2, update the UI and audio, and then redo all the sprites with good art direction as high rez painted style sprites (maybe modify the art style slightly over time if you're ambitious). Animated sprites. Nevertheless, 2D. Then just port in Civ 2 itself with minimum QoL adjustments, maybe AI. Then give as full as possible scenario/popular mod/feature pack. The motherlode. Anything that doesn't add or require new art.
Then, if an art asset gussie up on 4 with minimal changes and additions to gameplay or content, featuring the full expansion set, scenarios and popular mods (Ryse and Fall) is packaged with the 2 remaster I described and both look good, I think they could price at $70.
You'd be paying for completeness and prettiness, maybe some multiplayer facilitation. That might not seem worth $70, but if they properly redo the art with a qualified art direction team and full overhaul, I think it would be. I think it would be very popular too, for content creators too.
Imagine pretty little hundreds of animated civ 2 sprites fighting over territory, little nukes flying.