The fourth Avengers movie will have a different subtitle from Infinity War, but with the third movie still in pre-production it is not even in need of a working title.
Little is definitively known about Infinity War or the fourth Avengers movie, so it’s practically impossible to glean any sort of meaning from “Mary Lou” as its working title.
Production Weekly has reported (via ComicBook.com) that Avengers: Infinity War is operating under the working title “Mary Lou.”
Avengers: Infinity War, on the other hand, is already known as the name of the highly anticipated sequel, but now the production’s cover may have been blown.