Book 1 is in a class of its own. Books 2 and 3 are a little slower than 1. Book 4 is very slower as you say.

Book 4 brings out the most different reactions: some people even prefer it to 1, some just find it too slow. If you don't like the beginning, I don't think you'll like the rest as it doesn't change its pace. Maybe the problem is reading it in quick succession after 1, 2 and 3. The first time I read it it was after a break from the other 3 and it turned out I now love it (though I still prefer 1). If you're finding it frustrating a third of the way through, I would recommend dropping it now. Then if you feel like it, maybe restart after some time, maybe then it would work, and maybe it wouldn't.

Then books 5 and 6 pick up the speed once more, I find them very fast paced.

God Emperor is the 2nd best Dune book, so read it at least once.

Yes, very worth it. I've read all Frank Herbert's books and short stories. The last two Dune books, IMO, are the best things he wrote. It seems like he figured out that the Bene Gesserit were the most interesting thing he created, so he geeked out on them for two books.

Book 4 is very special to me as it's so strange.

5 is back to action!

To reiterate the other reply to your post, God Emperor of Dune is the 2nd best book in the series. It really pushes the envelope on imagining what the final outcome of a religion might be. It's a little polemic-heavy, but it IS a god we're talking about. :) Book 5 is the sleeping pill. After the frenzy that is 1-4, 5 just plods along in a dense aftermath.