In Fallout 3, when a companion died, they stayed dead, and they were quite prone to dying. This included the protagonists canine chum, Dogmeat. New Vegas was less cruel to companions, only killing them off in Hardcore mode, while just knocking them out in a regular game.
It’s not clear if that will remain the same when it comes to the Fallout 4 storyline, but Todd Howard has at least confirmed that (new) Dogmeat, who we saw in the trailer, will be immortal.
From @BethesdaStudios' Todd Howard: Dogmeat cannot die #Fallout4 #BE3 pic.twitter.com/0hSaF42xoD
— Bethesda Softworks (@Bethblog) June 16, 2015
An immortal dog, what a thing. I’m relieved. Kill as many humans as you want, Bethesda, but don’t touch a hair on that dog’s hide. I can’t handle dead dogs. I recall myself welling up when I was playing Call of Duty: Ghosts and had to carry a wounded Riley in my arms. I was also close to tears simply because I was playing the terrible Call of Duty: Ghosts, but that’s not really the point.
So there you go! Dogmeat can fetch stuff for you, follow you about and never die. Excellent.