Marking server as zombie due to 4xx response is odd

In LBHttpSolrServer.request, a request is retried and server marked as zombie if the return code is 404, 403, 503, or 500, and the comment says "we retry on 404 or 403 or 503 - you can see this on solr shutdown". I think returning a 503 on a shutdown is reasonable, but not 4xx, which is supposed to be a client error. But even if this is can't be fixed systematically on the server-side, seems like on the client side we can retry on another server, but not mark the current server as dead, because most likely when the server returns a 403 (Forbidden) or 404 (Not Found), it's not because it's dead.