There seems to be a lot of confusion gravitating around the Hardcore Prelude's rules about permadeath. The current disclaimer would have you believe once you exceed level 4, you no longer have to worry about permanently dying if you ever fall below level 4 again. Seeing as this isn't true and someone can just have a bout of bad luck days, weeks, or even months after doing the hardcore prelude and end up permanently dying that's probably something that would deter a lot of people from doing the Hardcore Prelude.
This is a bug or otherwise unintentional as far as I am aware. The Hardcore prelude is simply meant to be that--a more difficult prelude, that should you die during, you cannot get up, with level 4 as the cap for it. It is simply an unfortunate side effect that when these PCs fall below the level range after the prelude that they require a DM to get them back up.
Side effect or not I don't believe it'll ever be fixed so it's worth mentioning on the disclaimer.
I'd like to concur with this since I lost a hilarious character to it