404 is your best response. According to the HTTP RFC, http://www.ietf.org/rfc/rfc2616.txt,
A 400 Bad Request means:
The request could not be understood by the server due to malformed syntax.
Whereas, 404 states:
The server has not found anything matching the Request-URI.
The entire URI is your resource identifier, and you’re not finding a matching resource for that particular identifier.