There's a fairly accepted school of thought that says that there's no way the end of Avengers: Infinity War won't be entirely undone when the sequel comes along next year. There's just no way that many characters will stay dead when so many of them are key to the future of the MCU.
But the writers claim otherwise. They claim that the deaths are real and we all need to move on, as they told BuzzFeed News:
Markus: [Avengers 4] doesn’t do what you think it does. It is a different movie than you think it is. Also…[the deaths are] real. I just want to tell you it’s real, and the sooner you accept that, the sooner you will be able to move on to the next stage of grief.
McFeely: Put it this way, I think [Infinity War] is a fairly mature movie for a blockbuster. It’s got a lot of fun in it, obviously, but boy, it gets very mature. The second one is also mature. We’re going to own these choices, and hopefully surprise and delight you and get you invested. It’s by the same studio, the same filmmaking team. They were written at the same time, shot at the same time. They’re clearly connected, but they are definitely two different movies, one of which is dependent on what happened previous.
To a certain extent, of COURSE they'd say that, because they're protecting spoilers, but it's important to look at what they're really saying there. It might not be the film we're expecting, but that doesn't mean the snap won't be undone - it just doesn't also mean that everything will go back to being the way it was.
In other words, there will still be stakes and there will still be surprises. One of them might not be that the snap isn't undone, necessarily, but something else will come in place of that profound drama.