Fallout 4 designer Todd Howard said in a newly published interview that all of the player's companions - not just Dogmeat - are unkillable. There's a simple reason why they decided against permadeath for companions.
When an NPC companion takes a certain amount of damage, they'll be incapacitated. However, you can help them return to the fight by using a stimpack on them. Howard said in the interview with Bethesda.net that they felt companion death disrupted the flow of the game because players would always restart the fight so they could keep their ally alive:
It's a reload scenario. No one plays and the dog dies and they say, "Fuck it, I'll just keep playing." So we wanted to make him...when he's your follower, your actual companion in the game - there's a number of them, not just the dog - they can't be killed when they're in that state.
That was definitely how I played in Fallout 3. Whenever my companion bit the dust, I'd reload the game. This happened a lot, thanks to the NPC's tendency to walk off high ledges. It became such a chore to restart fights that I decided against taking a companion altogether. I'd imagine many players did the same. Even though the characters didn't have that much to say, I always felt like a dick letting them die. Plus, they were carrying a bunch of loot for me.
Bethesda doesn't want players to avoid companions. In fact, they want players to have stronger relationships with these companions than they have in past games. You can romance any human NPC that you travel with, regardless of gender. I guess they figured letting players put all this effort into courting them, only to have them get shot in the face by a raider, would be too cruel.
It's great that Dogmeat can't die because he looks more life-like than ever in Fallout 4. For this game, Bethesda based the canine on level designer Joel Burgess's German Shepherd, River. You can see them recording the dog's motions and sounds in this new behind-the-scenes video:
Bethesda said that there will be "around a dozen" companions you can recruit in Fallout 4. These include the minuteman Preston Garvey, a woman named Piper, and the robotic butler Mister Handy. The other companions haven't been revealed just yet but I'm sure we'll meet at least a few more before the game launches worldwide in November.