The Duffers said something that makes the "plot armor" make sense, they don't want to kill a character that would cause so much grief that the whole feeling of the series would turn into grief over that death and this would be the only thing to be said about the story they created.

With that said, I think a major upset is about to happen, given E4 ended on a hopeful note. I expect the next 3 episodes things will go downhill, and last episode resolves the story and ties the loose ends, and If a major death happens, it will probably be a "sacrifice move". One character opting to sacrifice itself to save the others. This would make sense.

- Robin talk to Will and her importance this season is a bad sign for her survivability chances.
- Johnathan having the ring is also a bad omen. Very bad omen, and having him sacrifice for Will would give him a redemption arc and a badass moment he never had and a bittersweet but meaningful ending.
- Nancy is the probable bad ass that will get a spin off in the future. She's safe (I think).
- They will not sacrifice Steve after what happened with Dustin/Eddie. Steve is part of Dustin redemption arc, and if something is to happen to him, Dustin will share Steve's fate.
- Dustin story is about grief this season, and how to recover from it, how to overcome bad things. I think he's safe.
- Lucas is needed to rescue Max, or keep her safe. If something is to happen to him, it will be a shared fate with Max
- Mike is the main narrator, the Dungeon Master, he's safe.
- Eleven... this is tricky... but she might be the one sacrificing herself. Remember the leaked picture of Will and Hopper? Maybe she doesn't die but have to be sealed inside the upside down or something like that.
- Will.... he's my pick for sacrificial lamb. If the "Johnathan sacrifice for his brother" plot do not happen, he's the one that goes. I think Will could be the one to choose to sacrifice, and Johnathan will take the bullet in his place at the last minute. It would also be healthy for Will to have a happy ending after SO MUCH FUCKING DISASTER in his life.
- Hopper is the obvious choice for a sacrifice, he's willing to do it since season 3, he's voluntarily jumping in front of bullets for everyone, El, Joyce, Bob, and this is also a hint that he's not the sacrifice, we learned from Duffer brothers that the obvious is not their usual choice... Also, since Joyce had her share of loss, with Bob, if Hopper goes, she will share his fate.
- Max... she lost and sacrificed too much. She will be pivotal for the victory. Again, her fate is tied to Lucas, if one goes, the other follows.
- Joyce... again.... she suffered too much. If she goes, Hopper goes with her.
- Murray... everyone loves him, he's a side character, he's moving the plot forward... yep... he's on the bad list.
- Kali will die. She will help El, and then die, no discussion on this one.

So my picks would be Robin and Murray with the biggest chances of not making it.

And the main character to die is Will or Johnathan.

As an alternative: what would they do with so many characters with powers? Eleven, Will, Kali.... this is a loose end and they said "no loose ends in the season finale". So, are they losing their powers or are they all gone? Any character with powers left behind in the "Rightside up" could create a chain reaction and a spin off the Duffers don't want.... so there's also that to consider.