I was asking about Workshop4. It seems now like this "Workshop4" thing is a separate application. I though it might have just been a sketch you were trying to compile with the Arduino IDE. I have no experience with whatever 4duino is.
When you install a hardware package via Boards Manager it is installed to a different location than the folder used for manual installation. It's possible that this Workshop thing is looking for the installation in the Boards Manager location only. What happens if you click "Yes"?
It should be possible to do a manual installation to the Boards Manager installation location if that is necessary.