There is a reason there are so many great movie trilogies out there ... and not so many great fourth movies. The general assumption is: anything beyond three (and I'm not including 'cinematic universes like the Harry Potter and Marvel movies), and it all starts to get a little tired and repetitive.
This is especially true when it comes to animated features – just look at the multitude of Ice Age, Madagascar, and Despicable Me movies out there. But as long as a franchise is still making money, you can always trust the movie studios to keep milking a property until it is dry.