Just like Mike French stated in his "Third Wheel" writing, fourths don't always work. I think these guys realize that and are getting ready to begin a new trilogy by tying the 4th to the 1st.
If it ain't broke, don't fix it. I think the "big 3" are all at a point in their careers where they should do something that works, makes them happy, and makes people happy. It seems Steven and Harrison had so much fun last summer filming this movie that they more than ready to Laissez Les Bons Temps Rouler!!!!!