There is no comic 404. Randall didn't miss an update either. He just skipped from 403 to 405 without saying anything. Now it would be very easy for me give this comic the worst grade ever for not being a comic at all, but that is really not how I feel.
I really like the little bits where Munroe includes a joke where you would not expect there to be a joke. He did that a lot with his alt texts, little jokes in his infographics, and now he does it a lot with his 'What-if' site, so much so that it's at risk of losing its effectiveness.
But that's where the essence of the joke lies. Humour relies on subversion of expectations. If you were expecting someone to make a joke (ever Monday, Wednesday and Friday) then it is very hard for that joke to be funny. In a context where you would not expect a joke, any joke becomes funnier.
This gets a rare A for humour, but an F** for messing up the numbering system such that idiotic forumites just have to point it out every time Xkcd reaches a landmark number. EVERY TIME.