So my first ever civ was civ5 base game and I was very new to strategy games in general so I only played on very low diff levels for 100hr or so. I then got civ6 which i truly got the next turn bug and have 1000 hours in.
Ive since gone back and played more of 5 and recently played a bunch of civ4 (after initially being put off by the UI/Graphics).
So I find the comparisons between the games very interesting. Personally from what I can see is that 4,5,6 all have pros and cons and are superb games in there own right. However I see a lot of bashing of 5 and 6 from players who love 4 and threads like this that state civ4 is the best civ game even now.
Civ4 is superb but honestly its not a better game than 6.
Combat is very simplisitc compared to 5/6 which I suppose the AI is better at but as a player its pretty boring. Mass out the same untit in one stack, go to city. Take city bashing the right mouse button done. Really not as satisfying as 5/6 combat.
City management is fun in 4, better than 5 but in 6 the terrain is much more interesting with district/wonder placement. In 4 you then basically spam cottages in a few cities, spam mines in a production city etc.
I like the commerce mechanic. Makes balancing your economy much more challenging and fun. Compared to 6 where gold comes very easy and then mid game you start to buy everything up.
UI - I hear so much that civ6 UI is terrible. For me 4 is horrible UI. Took me ages to find things and work out what was going on - much wiki reading. I get its a 2005 game so a different era and so I reckon if you played it back then you were used to it but in 2022 its not a better UI at all.
Civics - Civ6 is better and a lot more interesting at timing things but its the same concept 4 had so I like it. I didnt like 5 where you essentially had to lock yourself into a civic path and never change.
Tech - Tech tree in 4 is far better and more interesting compared to 6. 5 is decent as well and i think its because in 6 they moved a lot of things to the civics tree which wattered it down a lot. I usually use mods to improve it in 6.
Religion- This is very well done in Civ4. I like the diplomatic consequences as well as the less micro version of civ6s religion.
Overall for me Civ6 is still more fun but there is a lot to like about 4 and its been interesting learning how to play it. You can see lots of things that moved from 4 to 6 and then were devleoped further there and overall i think both are better than 5 which seemed under developed in many areas.