I'm not sure I would be able to give you many reasons why to play Civ6 instead of Civ5. But when asking about Civ6 versus Civ4, then I'm 100 % for Civ6. I don't remember Civ4 so well anymore (it's been quite a while), but just out of my head some "random" points against Civ4: Square tiles. Hexes are so much better, more logical, natural and mathematically interesting. Moving diagonally on sqares means that 1 tile movement is in fact 1.4times longer than horizontally or vertically. 3tiles radius of a city is quick to understand and makes sense, while in Civ4 you had to remember the exact shape of city's tiles. Stacks of doom. After moving to Civ5 I realised how strange the combat system was in all previous Civ games. Sure, 1UPT has some problems and disadvantages, but offers much more tactical options in fight and with clever use of terrain you can win against a much stronger army. In Civ4 you also could apply some tactics and strategy, but size of the stack was still extremely important. Too much micromanagement with all the Civ4 datadisks. The game in the end contained imho too many functions with no strong positive impact on the game. Factors like health, happiness, food, production etc. - all per city and all managed almost the same way (tile improvements, buildings, wonders...). Together with religion, corporations... (Yes, I know, Civ6 is a micromanagement hell, but that's because of a bad UI, so far...) Railroads giving +1 production - it was stupid not to use this, but it was equally stupid and tedious to build railroads on every single tile. I wasn't a fan of the idea where a city with no military unit could not defend at all and even the weakest enemy unit could capture it in one turn. I think at least some citizens would fight for their city And even when we put realism aside, it was bad from gameplay point of view.