Basically if they put EU3 into the EU4 engine, I'd be thrilled, because EU4 is much, much prettier to look at than EU3, but I have fun with EU3 nearly the whole time from start to end, I've played many games to the end there, but the only similar amount of fun I have in EU4 is in the first 50-100 years of the game and I have never finished a game in EU4 other than in observer mode.
I played to 1705 or so once and just said ahh, bored and let the game finish it. It did a remarkably poor job with my country that I'd left it, but that was more fun to watch than if I'd continued. I keep wanting to do the 3 mountains, or even just a WC for the bling of the achievement on Steam, but since I am not a bling hunter naturally, my desire does not outweigh my dread of the tediousness of the experience, so I haven't done them yet.