Yeah, Herbert took a detour with books 3 and 4. I'm not sure which one is worse out of those two. I'm inclined to say it's book 3 because all I can remember is how crazy abusive everyone was toward Leto II; nothing really happened after they started doping him up on spice. I just finished reading Paul of Dune by Brian Herbert and Kevin Anderson. It's a direct sequel to Dune, and it shows how Paul went from savior to tyrant as he tries to conquer the old Imperium, develop a religion around himself, and maintain order.
How did you manage to persevere through book 3? Even with the cool prospect of Alia being taken over by the Barron, the series turned awful. The characters became far more conniving, but it seemed contrived. The plot was dull. Leto II didn't come off with the charm that Paul did.
Do you recommend Brian Herbert/Kevin Anderson's novels on Dune? I avoided them on the basis that derivative works, even based on the author's notes, are terrible.