Immortality is possible in Diablo 4. You just need the right build and a little ring that might take a while to get, but it's possible, as demonstrated by YouTube user Emilyheartz987.
In one of Emilyheartz987's recent videos, a max-level Butcher spends 20 minutes trying to kill them as they stand there without attacking. Their health bar doesn't budge as one of the hardest bosses in the game tries to cleave them down. Emilyheartz987 is so confident that they casually tweak their skills as the one-sided fight goes on.
Emilyheartz987's Mana Tank Sorcerer is what I'd call a variant of the popular Ball Lightning Sorcerer build, which is easily the strongest build in the game right now. Ball Lightning relies on a Legendary Aspect that lets you surround yourself in absurdly high damage electric orbs. They cost a ton of mana, but with the right gear, you can sweep through the hardest dungeons in minutes, and kill bosses in seconds.
The Mana Tank Sorcerer build retains the swirling orbs of death but stacks loads and loads of armor and damage reduction to trivialize hits from high level enemies and combines that with the Melted Heart of Selig Uber Unique amulet. You can farm the Uber Duriel boss and pray that you get it quickly. When you take damage with the ring on, your mana is hit instead of your life. By significantly reducing the amount of damage you take and amplifying your mana regeneration, monsters simply can't hit you hard enough to fully deplete your mana, making you invincible.
The Butcher 1v1 isn't entirely representative of the build's potential. You can't go fully AFK if there are enough enemies attacking you, their combined damage can outrun your mana regeneration. But all you need to do is cast a few spells that spawn crackling energy on the ground to keep it topped off.
Damage is the only sacrifice you have to make, but Ball Lightning can still carve through normal enemies pretty fast. Bosses, however, take a while to kill, so Emilyheartz987 plans to use a super powerful Glyph from the upcoming Abbatoir of Zir endgame dungeons to make up the difference. Blizzard says even the most powerful builds will struggle in the Abbatoir of Zir, which will function like Diablo 3's Greater Rifts. Enemies will scale even higher than the max-level Nightmare Dungeons and you'll only have a few minutes to finish each tier. The Mana Tank Sorcerer is built specifically for that kind of challenge and I'm going to go work on copying Emilyheartz987's build before it drops on December 5.