Is Drunk disguised as Knight supposed to deal an additional 4 damage when executed, or is that just an unintended side effect of Knight dealing 4 extra damage when corrupted? 
I have searched for that card outcome, and I can type that yes it is intended.
The reason why is simple, I believe that the corruption hint for the corrupt Knight is allowed to work for the Drunk due to:
A execution mechanic: When a Disguised Drunk as a Knight is executed, the game registers the Corrupted Knight's hint and drains 4 Hp, than it reads the Disguised Drunk's hint and drains 2 Hp.
1 more thing, the two mechanical parts are both hints, The Knight's hint and the Drunk's hint.
You can notice that some characters have hints, like if a Empress is lying than all 3 are good, and more like that, I think hints aren't really hints but more of mechanics that are read by the game and used along with abilities.
(The edit is me adding "hint" after I realize that my previous explanation was over disguises when really it's just the hints)