Josh Klausner, one of the writers of Shrek Forever After, explained in 2010 the script's evolution: "When I first came onto the project, it wasn't supposed to be the final chapter — there were originally going to be 5 Shrek movies. Then, about a year into the development, Jeffrey Katzenberg decided that the story that we'd come up with was the right way for Shrek's journey to end."[6]
In February 2014, in an interview with Fox Business Network, Katzenberg hinted that a fifth film may still be made. "We like to let them have a little bit of time to rest," he said of the characters. "But I think you can be confident that we'll have another chapter in the Shrek series. We're not finished, and more importantly, neither is he."[7]
On June 15, 2016, after NBCUniversal purchased DreamWorks Animation for $3.8 billion, NBCUniversal chief Steve Burke discussed plans to revive the franchise, as well as other DreamWorks films.[8][9] In July 2016, DreamWorks Animation (DWA) announced that the fifth film will be released in 2019.[10]
TL;DR there was always a 5 movie plan
