4th season will be the last. The last renewal we heard of was over a year ago, and the article about the show switching channels just mentioned that renewal as though it was a new thing (meaning there's no upcoming season to announce).
Also a bunch of people on the crew, ex. Sabrina and Zach, are going on to work on other Disney shows. Disney likes to keep people who worked on previous shows working within the studio if they can (ex. Sabrina and Amelia both worked on Gravity Falls before Star), meaning their work on the show has finished or is about to and so they're moving on to a new project.