In 4e, you could do the same thing, but if you want to give the PCs a chance to live, use the encounter as a skill challenge. You could even make it a level 1 (or whatever is appropriate) challenge to give the PCs a chance and work the big bad guy into the story.
For instance, the PCs open a door to a summoning chamber in the abandoned wizard's lab, and out bursts Demogorgon. Before he returns to the Abyss, his two heads demand that the PCs do him a favor. Cue the skill challenge (success, you're in Demogorgon's debt; failure, he eats a few PCs).
There's no reason why the superstars of D&D's monster world can't show up early in a campaign, and the skill challenge system is a good way to use them in situations other than combat.