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

Is it a valid request? Can the id of the resource exist? Is it formatted as a proper id? Is it syntactically correct? etc.. If so then you can use, 404 Not Found. Otherwise 400 Bad Request is more suitable.