Nice one from Ifor! However I can't parse 4D, though I've reduced the possibilities to three, none of which bear any resemblance to anything defined in the clue. The highlighting is straightforward though.
I'd be very surprised if anyone highlighted the red herring - you'd have to ignore all of the letters and focus entirely on the numbers for that to happen... oh hang on a minute (Yes, entirely possible, sorry).
Thank you, Ifor. A challenging grid fill and an entertaining endgame. I particularly liked the way that the final letter changed simultaneously produced both WHO and not-WHO in different orientations. Very clever!