[WayBack] xcopy | Microsoft Docs has this:
Exit code Description 0 Files were copied without error. 1 No files were found to copy. 2 The user pressed CTRL+C to terminate xcopy. 4 Initialization error occurred. There is not enough memory or disk space, or you entered an invalid drive name or invalid syntax on the command line. 5 Disk write error occurred.
Emperically, errorlevel 4 is also returned when the source file or source directory does not exist.
–jeroen