I can confirm that videos, articles, images, 404 pages are all served by a CDN. Our 404 pages were not cached by the CDN. Allowing them to be cached reduced the origin penetration rate substantially enough to amount to a 66% reduction in outgoing bandwidth over uncached 404s.

Edit: This is not to say that our 404s were not cached at our origin. Our precomputed 404 was cached and served out without a database hit on every connection, however this still invokes the regular expression engine for url pattern matching and taxes the machine's network IO resources.