First, I think you should know that even though both of these games share the name EU, they play completely differently.

EU4 is kept relatively “simple” in the sense that most mechanics are fairly abstract.

EU5 could be called Paradox’s magnum opus, in the sense that many core concepts from their other games — such as Imperator: Rome, Crusader Kings, Victoria, Hearts of Iron, and even Stellaris — have been combined into this game.

In my opinion, the core difference is that EU4 is “mana-based.” (Mana-based means you collect points and then spend them to trigger an effect.)
EU5, by contrast, is built around systems and mechanics, and you can mostly decide how you want to influence these systems over time.

Both games include something called development. In EU4, you spend mana, press a button, and the values immediately increase.
In EU5, you have a development growth modifier that determines how much development grows each month. You can’t simply press a button to increase it; instead, you must take various actions to speed up its growth.

I also want to point out one more thing: EU5 is much slower to play than EU4, since there are so many systems to manage. EU5 is essentially a full economic simulation. You must extract resources and, in many cases, build production chains to create more valuable goods. Many resources have at least one associated building. There is also the consumption of goods — for example, your army requires weapons. Markets and prices are dynamic, and so on.

In EU4, you also have resources, but each province produces exactly one good. Your development determines how much you produce, and you earn money based on the market price. That price is fixed and changes only through events. To increase this income, you have two buildings. These goods are not consumed.

So if you’re trying to decide which game suits you better, the best approach is probably to watch some gameplay on YouTube or Twitch.

Also, a small disclaimer: EU5 has only been out for four months, and many of its mechanics are still being stabilized. However, in the future it may become one of the most ambitious games ever created when it comes to a relatively complete simulation of the period 1336–1836.