There are some descriptions here: https://developers.google.com/cast/docs/reference/ios/g_c_k_error_8h#aea7a716be62f301015e255e1ba63a9cc
Error code 4 you mentioned looks like it means that an invalid request was made.
If you happen to be programming for iOS, in the GCKError.h file, I found there are some additional error codes:
typedef NS_ENUM(NSInteger, GCKErrorCode) {
/**
* Error code indicating a network I/O error.
*/
GCKErrorCodeNetworkError = 1,
/**
* Error code indicating that an operation has timed out.
*/
GCKErrorCodeTimeout = 2,
/**
* Error code indicating an authentication error.
*/
GCKErrorCodeDeviceAuthenticationFailure = 3,
/**
* Error code indicating that an invalid request was made.
*/
GCKErrorCodeInvalidRequest = 4,
/**
* Error code indicating that an in-progress request has been cancelled, most likely because
* another action has preempted it.
*/
GCKErrorCodeCancelled = 5,
/**
* Error code indicating that the request was disallowed and could not be completed.
*/
GCKErrorCodeNotAllowed = 6,
/**
* Error code indicating that a requested application could not be found.
*/
GCKErrorCodeApplicationNotFound = 7,
/**
* Error code indicating that a requested application is not currently running.
*/
GCKErrorCodeApplicationNotRunning = 8,
/**
* Error code indicating the app entered the background.
*/
GCKErrorCodeAppDidEnterBackground = 91,
/**
* Error code indicating a disconnection occurred during the request.
*/
GCKErrorCodeDisconnected = 92,
/**
* Error code indicating that a request could not be made because the same type of request is
* still in process.
*/
GCKErrorCodeDuplicateRequest = 93,
/**
* Error code indicating that a media load failed on the receiver side.
*/
GCKErrorCodeMediaLoadFailed = 94,
/**
* Error code indicating that a media media command failed because of the media player state.
*/
GCKErrorCodeInvalidMediaPlayerState = 95,
/**
* Error code indicating that the application session ID was not valid.
*/
GCKErrorCodeInvalidApplicationSessionID = 96,
/**
* Error code indicating that an unknown, unexpected error has occurred.
*/
GCKErrorCodeUnknown = 99,
};