What is going wrong when Visual Studio tells me "xcopy exited with code 4"

Xcopy exit code 4 means "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."

It looks like Visual Studio is supplying invalid arguments to xcopy. Check your post-build event command via Project > Right Click > Properties > Build Events > Post Build Event.

Note that if the $(ProjectDir) or similar macro terms have spaces in the resulting paths when expanded, then they will need to be wrapped in double quotes. For example:

xcopy "$(ProjectDir)Library\dsoframer.ocx" "$(TargetDir)" /Y /E /D1

Switch the watch tab to the "ouput" and look for the xcopy command. Sometimes here you find some more message ( the actual xcopy output ) that could help you to solve the issue. If you don't see the output tab, use View-Output menu to show it.

I addition to the accepted answer, the error can also occur when the destination folder is read-only (Common when using TFS)