You know how in Seasons 1, 2, and 3 you saw characters like Mack and Madison? Here is the reason Season 4 won't showcase them.
Back in those seasons, they were at Edgewood School. Now that they are in High School they all went to New schools. It would be illogical if they were all at the same school. This is the reason why you don't see Mack and Madison. Another reason you don't see some of them is because they became stars on Disney or quit acting. For example, Lincoln Melcher (Mack) and Sofia Wylie (June) both haven't appeared since before they started filming on Disney.