Clarifications:

* At time of writing (2025 March), Avengers Doomsday is scheduled for 2026 May 1.

* TV shows do not count wrt this question.
* If Avengers Doomsday is delayed or reworked & a different similar film is released first, that counts.

* Such a film must be titled 'Avengers' or be about Avengers characters. For example, Captain America Civil War would have counted.

* Specifically, 4+ of its main characters must be heroic main characters from previous Avengers films.

* It's not sufficient for these 4 characters to be secondary or cameo roles alongside non-Avengers main characters.

* If the copyright situation changes a lot, non-Disney Avengers films could count, assuming they are still big-budget visual quality.