A 4xx error is a developer-assigned error. Common 4xx errors are 404s (Not Found) and 400s (Bad Request). Technically, although it's bad practice, developers can assign a 4xx response to a "normal" app function, hence URLs can have a high percentage of 4xx's and not be concerning to the developer. Therefore, high 4xx counts on a URL are not always "bad."

A 5xx error is a server error. Spikes in 5xx's can represent a server outage and thus, 5xx's are almost always "bad."