As we've known for more than a year now, there's a very good reason why we don't yet know the Avengers 4 official title. Kevin Feige has already spelled it out pretty well: the title is a spoiler for the events of Infinity War (and it's not as simple as it being something like "Infinity Gauntlet", apparently).
Naturally, that's made everyone anxious about the possibilities, since the most likely answer is that the title will refer to the fate of some of the heroes. So something along the lines of Avengers: Disassembled or Avengers: Iron Man Is Definitely Dead or something like that. And while you could have pinned that down to natural pessimism, the directors of Infinity War have now confirmed that the whole spoiler title mystery has been precisely designed to scare us all.
Here's the ominous way the Russos responded to a meme-tweet worrying about the scary implications of the title being suppressed because it is a spoiler...
They literally want us to worry. Nice.
It's still unclear just how many heroes will be sacrificed in Infinity War (if any), but it's also still quite unlikely that the title will directly reference a single dead character (even in the wake of Infinity War's release it would be too spoilery) or even something like the reason why the Avengers travel back in time. So it's intriguing to think of what the possibilities could be.