Bolded for emphasis - mine.
You cannot even imagine it coming up and yet it did for your example for non-4e D&D? If 4e treats it the same as any other edition by looking at the stats then I see no difference.
I talked about
how it is that we know a 1st level fighter can't beat a pit fiend. In 4e, as I said, I know that from the fiction - a character at the bottom of the Heroic Tier is not going to succeed in the sorts of adventures that are described (in the PHB and DMG) as pertaining to Paragon and Epic Tier heroes. Eg from p 29 of the PHB:
In the epic tier . . . [y]ou navigate otherworldly realms and explore neverbefore-seen caverns of wonder, where you can expect to battle savage pit fiends . . .
Whereas p 28 tells us that, in the heroic tier,
The fate of a village might hang on the success or failure of your adventures, to say nothing of the risk to your own life. You navigate dangerous terrain and explore haunted crypts, where you can expect to fight sneaky goblins, savage orcs, ferocious wolves, giant spiders, evil cultists, and bloodthirsty ghouls.
Such a person has no hope against a pit fiend.
You asked "what would you do in 4e with a 1st level PC attempting to strike a Pit Fiend?" And I answered that I can't imagine it coming up. Therefore I don't need a theory of how to resolve that action declaration. But I don't need such a theory to know that the 1st level fighter would have no hope, as the fiction tells me that.
I will repost LostSoul's post that makes this point as clearly as I believe it can be made, about the sequence of fiction and mechanics in 4e compared to other editions (he focuses particularly on 3E, but I think the point generalises):
How the imagined content in the game changes in 4E as the characters gain levels isn't quite the same as it is in 3E. I am not going to pretend to have a good grasp of how this works in either system, but my gut says: in 4E the group defines the colour of their campaign as they play it; in 3E it's established when the campaign begins.
That's kind of confusing... let me see if I can clarify as I work this idea out for myself.
In 3E, climbing a hewn rock wall is DC 25. That doesn't change as the game is played (that is, as fiction is created, the game world is explored, and characters grow). Just because it's DC 120 to balance on a cloud doesn't mean that characters can't attempt it at 1st level; they'll just always fail. The relationship between colour and the reward system doesn't change over time: you know that, if you can score a DC 120 balance check, you can balance on clouds; a +1 to your Balance check brings you that much closer to success.
In 4E, I think the relationship between colour and the reward system changes: you don't know what it will mean, when you first start playing, to make a Hard Level 30 Acrobatics check. Which means that gaining levels doesn't have a defined relationship with what your PC can do in the fiction - just because your Acrobatics check has increased by 1, it doesn't mean you're that much closer to balancing on a cloud. I think the group needs to define that for themselves; as far as I can tell, this is supposed to arise organically through play, and go through major shifts as Paragon Paths and Epic Destinies enter the game.
In 5e we have lair and legendary actions/saves to assist with the solo mechanic.
But is the legendary status part of the fiction? Or just a device. In 4e solo status is just a device, for ensuring that the mechanics match the fiction which is known
independently of the mechanics. That's whty the very same pit fiend can be statted as a solo or an elite, depending on the level of character opposing it. And why the very same goblin can be statted as a standard, a minion, or subsumed into a swarm.
You certainly structure challenges on tiered feasibility which is really no different than how the monsters have been compiled with challenge ratings in their MM. CR is a sort of tier.
So what is the appropiate DC for a non-cbombat hallenge that is "tiered" to be feasible for a 15th level fighter but is out of the question for a 1st level one?