My previous experience
I have previously owned and played TI3 a handfull of times and I had mixed feelings about it. There were so many little things in the base version that I didn't like. Sure most where later fixed with the expansions. In TI4 you get a definitive version of everything.
Why TI4 is better
TI4 is just a much better game because it fixes all issues I had with TI3 and streamlines a lot. I like the new strategy cards, how trading works, the overhauled technology tree. I didn't like Distant Suns or the Leaders so I don't miss them at all.
More stuff I like
I like getting the 17 races with 2 racial techs, the flagships and promissory notes in the box. The new insert isn't perfect but at least it is usable. The public and secret objectives also breaks up the traditional turtleing. And finally, I personally think the new artwork is fantastic and looks way more modern.
What I don't like
TI4 is streamlined but it is still a game that takes a long time to play and has a lot of rules that can be quite complicated for new players. Once you get into the second half of the game it will start to feel more and more like a slog.
Not for everybody
TI has never been a game for everybody and TI4 doesn't change that. It's still a long epic space war game where you roll dice to decide the outcome of battles and the luck of card draws has an impact on the game. It is still a game that will likely take a whole day and is difficult to get to the table.
For me personally, I like this version and I will keep it and try to get it to the table more often. It is the best epic space experience out there. And I think it's worth the money, if it's the type of game you like.
Played my first game of TI ever this past weekend and it didn't disappoint. I totally understand now why this game has lasted the test of time.
Awesome fun with every turn filled with action, adventure and surprise.
Would love to play it again because it was truly an epic experience, but don't know when I will have another 8+ hours to spend doing it.