Should a RESTful API return 400 or 404 when passed an invalid id

Question

When building a RESTful API and a user provides an id of resource that does not exist, should you return 404 Not Found or 400 Bad Request

Answer 1

404 Not Found is the proper answer I think, 400 is more about the body of the requests and not the resource identifier, so for example you can send that by validation errors.