The initial plan was to film Avengers: Infinity War and Avengers 4 simultaneously. However, we learned some time ago that the movies will be shot back to back, with Infinity War shooting ending in June or early July 2017, and Avengers 4 beginning to shoot in August. So there’s nothing new on that front. However, when the Phase III movies were announced, the third and fourth Avengers movies were both titled Infinity War. That changed last year, and the last part (presumably) of the Avengers franchise remains untitled.

Kevin Feige explained that shooting the two movies back to back, rather than simultaneously, would be easier for all involved:

We're doing them one right after another"¦It became too complicated to cross-board them like that, and we found ourselves"”again, something would always pay the price. We wanted to be able to focus and shoot one movie, and then focus and shoot another movie (Collider).

But there’s more. It initially made sense to reveal Thanos' final plan across two movies, but now we can’t be so certain that it’s going to happen that way. And, as Feige said, Infinity War and Avengers 4 are two completely different movies. How different? We still don't know.

We do know that it’s a bit peculiar that the Avengers 4 title remains under wraps. Feige explicitly said (via Screen Rant) that it won’t be revealed for quite a while, because the title would spoil the ending of Infinity War.

We feel that this is great news, since we now have a surprise to anticipate until Infinity War hits the theaters. Sure, this goes against what we’ve become accustomed to from Marvel and Disney, but it also means that we’re stepping into new territory, and Infinity War will be a huge event.