Extras placed on the DVD releases of the films! Things like bloopers, behind-the-scenes content, games, trivia, and director's commentary.
Most of them have been uploaded to YouTube or the Internet Archive. They'll be embedded here!
NatM 4 was released on streaming only, making it the first digital-only entry in the series. It's very unlikely that we'll see any extra content about the film, as Disney won't even release ratings for their service. It's also the first animated entry, and bloopers in animation are often voice lines being flubbed - it actually costs a lot of money to animate bloopers! That's why Pixar's bloopers are some of the few that exist; they had the money to animate them.
However - we HAVE found some outtakes in some of the animation reels available on the Concept Art page.
A short released with the Blu-Ray of NATM 3, in which Ahkmenrah's friend Craig makes a mess of the set. Archived here.
Archived by FilmIsNow, has their YouTube outro at the end. Otherwise unedited.
Uploaded in three parts.