四开头的词语：401(k)；402(f)；403(b)；404 error；405 method not allowed；406 Not Acceptable；407 Proxy Authentication Required；408 Request Timeout；409 Conflict；410 Gone；411 Length Required；412 Precondition Failed；413 Payload Too Large；414 URI Too Long；415 Unsupported Media Type；416 Range Not Satisfiable；417 Expectation Failed；418 I'm a teapot；419 Authentication Timeout (not in RFC 2616)；420 Method Failure (Spring Framework)；421 Misdirected Request；422 Unprocessable Entity (WebDAV)；423 Locked (WebDAV)；424 Failed Dependency (WebDAV)；425 Too Early (Internet draft)；426 Upgrade Required；427 Unassigned；428 Precondition Required；429 Too Many Requests；430 Unassigned；431 Request Header Fields Too Large；432-499 Unassigned