When running 4D's default MDI mode on Windows, you may have noticed that hitting the top right "X" icon of the MDI window while a form is open does not close 4D when the form does not have an associated menu bar. This behavior is due to how the close action is linked to the quit associated standard action of a menu bar, but without the menu bar, the quit action is not linked to the top right "X" icon to close 4D.

To resolve this issue, simply assign the form an associated menu bar and 4D will be able to close while the form is open.