At the end of season 3, Ryan Murphy extended an open invitation to all 15 regulars to come back to the show in season 4, and according to Deadline Hollywood, it appears all have accepted it and are coming back, though not in every episode. Lea Michele and Cory Monteith are among those who most likely will be in every episode.
The workload will vary due to the size of the cast and to introduce new characters. In addition to storyline reasons, some cast members may have a reduced presence on the show as they pursue other projects.
(in other words, there is a chance that some of the cast may only be in one or two episodes - but that's just my thought).