When I started playing EU4, I thought that EU5 would be released "soon". That seems increasingly unlikely. In fact, it isn't even guaranteed that EU5 will come out before 2027. But even more importantly, my general ideal of "let's play some EU4, so I'll have an easier time getting into EU5" seems very flawed. From all I know about EU5, the core system is so dramatically different, that applying lessons from the previous game is probably not helping at all.

What I learned from watching those videos of different versions of EU4 is that much of the Europa Universalis gameplay is actually about managing various restrictions, and that these can change a lot even between patch versions of the same game. The current (and probably final) version of EU4 is very much about the management of monarch power points (administrative, diplomatic, military). In EU5 these have been replaced by monarch abilities in the same categories, and the progress of technology is now a more classic tech tree with research points, rather than using monarch power points. In fact, in some respects of game mechanics the previews of EU5 resemble Victoria 3 more than they resemble EU4. Maybe I should play that again, only that Vic3 unlike EU4 doesn't have the possibility to subscribe for some months and get access to all DLC content.

What I find remarkable is how little the gameplay of Europa Universalis 4 is based on anything "universal". There is a lot of "gaming the system" going on, and that very much depends on minor details of the system. When the system changes through a patch, or a new version of the game, there is no value anymore in knowing how to game the previous system. On the positive side, this probably added a lot to the replayability of EU4: Over time the devs have added a lot of local systems. The game mechanics applying to the Holy Roman Empire apply only there, and for example the Ming Empire has a completely different set of rules. So you are learning new systems and new ways to deal with those systems every time you play a new game somewhere completely different.

I'm still planning to buy EU5 on release, and am looking forward to playing that. But I don't think anymore that playing EU4 helped. And there is a strong possibility that learning to play EU5 at release will already mean a lot of those learned skills become obsolete by the time patch v1.1 is released.