I am little concern about GC4.
1. First it would have to include all features from 3 at lunch and be better at the same time
2. There is a lot of cool ships that would needed to be recreated not to mention mods.
3. If the game turns out to be just an improved version buying it again at full price might make no sense, maybe if it was as an upgrade from 3
4. You made a lot of improvement recently and all thou there is still place for polish/bugfix/typos I can't realisticly expect GC4 would be free from it from lunch.
This will probably be tricky decisions with a lot of technical/financial aspects to consider.
Personally I think there is still a lot of life in GC3 and I can't really see too much place for significant change in general gameplay (major thing holding game back is no doubt space combat, but personally for me is not that big problem) I hope you will not take same path civ6 took, making changes just to make changes (I can't even look at screenshots of civ 6 and I still think 5 is way better, just as I still sometimes see comments saying GC2 was better)
I once heard someone saying something like: "everyone got his/her civilization". Ppl still play Civs 2,3,4... A while ago I even gave a try 'Call to power 2' and 'Alpha Centauri'. New isn't always better in this genre.