4xx are client-side errors, so it's not the case.
5xx are server-side errors, so they're wrong too.
1xx seems like a good option, but then they MUST NOT have a message-body IIRC.
2xx seems like a good choice. The request was processed, understood, and accepted. It's just not the origin server that processed it.

Although, a mechanism for telling that the response has not been sent from the origin server is needed here. This could be done by a header with the hostname of the machine that is responding, like '"Processed-By" ":" host'.