Robots made $260 million worldwide; I wouldn't consider that "hardly a success". Now true, their Ice Age films have made more, but Robots didn't exactly flop. And I don't see why Horton shouldn't count simply because it was based on a book. You'd have to rule out a LOT of films from studios if that were the case.
Anyway, I just don't want the series to become The Land Before Time. 2 was a worthwhile sequel, 3 was a letdown but had a logical closure point; I'd hate for them to milk it, especially if they don't have any ideas.