Why was Jake and the Never Land Pirates cancelled? And seeing its open ending, is there any information of how the show would have ended or any plans for releasing some kind of resolution of its story?
At the same time that Jake and the Never Land Pirates weren't being renewed, a new series ("Mickey and the Roadster Racers") was in production. The show was touted as having the same 'creative talent' as Jake.
The Emmy®-nominated creative talent behind Disney Junior’s hit series, “Mickey Mouse Clubhouse” and “Jake and the Never Land Pirates,” comprise the production team: Rob LaDuca is executive producer, Mark Seidenberg is co-executive producer/supervising story editor, and Thomas Hart is story editor. The series is a production of Disney Television Animation.
Additionally, some 51 staff and crew (including all of the major voice talent and dozens of people from the art teams, writers, producers, sound recordists and animators) moved from one show to the other so the most likely reason for Season 5 of Jake not being produced was that the people who made it were now unavailable.
If I had to speculate, I would imagine that a combination of declining ratings and lowered merchandise sales convinced Disney that they wanted to keep the 'creatives', but ditch the property itself.
Head writer, Brian Swenlin seems open to the idea of a new season... at some point.
Q. Will Jake and the Never Land Pirates ever get a fifth season by any chance?
Brian Swenlin: There's always a chance! But, to my knowledge, there is not another season currently in the works.
According to the Jake and the Never Land Pirates' Disney Fandom Wikipedia Page, it was canceled "in light of Mickey and the Roadster Racers".
Disney executives announced the show was cancelled and would not receive a fifth season in light of Mickey and the Roadster Racers. The last episode aired on November 6, 2016.
Doing some more research I found that 'Jake's' writer Brian Swenlin is the one who moved onto Mickey and the Roadster Racers. I have not found sufficient evidence to conclude if 4 seasons of 'Jake' was always planned this way and/or any plans on what a potential season 5 would of been.