I think it's better to return the 404. I hate it when the Comcast DNS redirects me to their search page when I misspell a domain name.
Silent redirects are not so nice, but if you can see why you were redirected it shouldn’t be a problem.
https://dns-opt-out.comcast.net/
Best set of hoops I ever jumped through (well, with regards to Comcast, at least).