acceleration profile: HIT upon 404

I am assuming you are using webaccelaration profile, and not AAM.

Are you sure about that? Because I remember situations were site was down, but the servers were up. That was caused by caching, as one of the servers went down, and that page was cached. I think was 500 level code, but the same idea should apply to 400 level, as is not helpful to cache none of them.

I think your problem is more with the fact that the page is dynamically generated, so dynamic pages should never be cached. Check the headers you get when you receive that page, from the server to F5. Let me know if you need help in how to capture that.