At the moment we know virtually nothing about Avengers 4. The movie coming out next May and we don't have a title yet. We have some rumors and a few leaked set photos to go on. There have been a ton of rumors about the production but one of them is that time travel is going to be involved [possibly]. That means that old characters could come back to life. One of those characters of Pietro Maximoff aka Quicksilver played by Aaron Taylor-Johnson who has been dead since the events of Avengers: Age of Ultron. Taylor-Johnson spoke to Metro and was asked if there were any talks about him returning to the Marvel Cinematic Universe.
'It's not a conversation right now,' Aaron said.
So it sounds like there aren't any plans for the other Maximoff twin to make another appearance. We can hope that the Scarlet Witch TV show might address how Wanda is reacting to losing her other half. Even if the MCU doesn't have plans to bring Pietro back it would be nice if they at least addressed his death in some sort of way.
Or Taylor-Johnson is lying because the Marvel snipers are out there. We'll see in May.
Untitled Avengers Movie / Avengers 4, directed by Anthony and Joe Russo, stars Robert Downey Jr., Josh Brolin, Mark Ruffalo, Chris Evans, Chris Hemsworth, Jeremy Renner, Chris Pratt, Chadwick Boseman, Benedict Wong, Zoe Saldana, Karen Gillan, Vin Diesel, Scarlett Johansson, Don Cheadle, Sean Gunn, Brie Larson, Evangeline Lilly, Paul Rudd, Jon Favreau, Gwyneth Paltrow, and Ty Simpkins. It will be released on May 3rd, 2019.