Here's the breakdown:
* Season 4 (2013-2014) was the final season of new episodes aired on Disney Channel.
* "Phineas and Ferb: Star Wars" (2014) was a special movie-length episode released after Season 4, continuing the story.
* "Phineas and Ferb: The Last Day of Summer" (2015) was another movie-length special that acted as the definitive series finale.
* "Milo Murphy's Law" (2016-2019) was a spin-off series featuring a recurring character from "Phineas and Ferb," Milo Murphy.
So, you could say Season 4 is the last traditional "Phineas and Ferb" season, but the story continues in the specials. "The Last Day of Summer" is widely considered the true ending to the series.