It is possible to close all the windows in the Design environment by holding down the Option or Alt key while opening up a database.

Note - Opening a database while holding down the Option or Alt key, will first bring up the prompt asking if you would like to open the database with an existing or new data file. After making your selection, hold down the Option or Alt key again while 4D is starting and this will close all the windows.

If you are already in Design environment, all windows can be closed by selecting the menu item Close All Windows from the File menu, or by closing an open window while holding down the Option or Alt key.