So if done right a 4th movie could easily break DOTM's results. With great casting, story and director everything is possible.
Talking about casting - although I really loved Rosie in DOTM I think that if the movie had Megan instead, the movie would have made more money. I'm absolutely with Michael and Steven about this decision, it was the right one. Just I have a couple of friends who are not fans of the movies but watched them only because of Megan. When they heard that Megan won't return they refused to watch it. I guess there are plenty people around the world that did the same. Just look at Megan's fan page on Facebook $20+ million fans. You can imagine how much more tickets these are.
But still you can never predict such things. The movie is what it is - a great success sitting proudly (in 2 weeks time) on number 5 at the all time world wide box office.