Good question! The essay itself is expected to be in the 400-word range. We haven't literally heard this from Dee Leopold herself, however we highly doubt that the HBS adcom is likely to count the question that you're posing for yourself within that word limit. The question itself shouldn't be too long though! Harvard essay questions, as you know for yourself from working through the other three essays, are quite short. Don't be tempted to write a novel-length essay question such as Wharton has! So keep the question short, and then write 400 words from there.
(Note that HBS doesn't count words, so if you go overlimit slightly, it won't hurt you.)
Good luck with those essays!!! That deadline is certainly stressing a lot of people out this weekend. (We've been tweeting all day today with tips and tricks - will be doing so again on Sunday - we're also offering impromptu essay reviews to help you guys in the final stretch.)
EssaySnark