One of the biggest theories you see online on places such as Reddit, is the belief that Avengers 4 will be titled “Avengers Assemble”. There’s just one problem with that, Marvel has already used that title…for the first Avengers movie.
In the UK there was a 60’s television series (and really bad 1998 big screen version) called The Avengers. The show is known in the US and other parts of the world (which is why the bad 90s movie was an American production), but it was really best known in the UK.
Because of that, when Marvel was preparing to release The Avengers in theaters in 2012, they changed the title of the UK release to Marvel’s Avengers Assemble so people wouldn’t confuse it with the 60s TV series or the terrible 1998 movie.
Whatever Avengers 4 will end up being titled, it will be close to “Avengers Forever”, but it won’t be Avengers Assemble.