Haven't used this feature before, but it appears when in the 'Edit Database Cell' dock, there is a button to edit the text in an external application (eg, Notepad, if on Windows).
However, it wasn't possible to save this text within Notepad - the file was locked (I assume by DB4S...)

Screencast: https://streamable.com/5bbtoj

So I clicked the button, edited the file, but when you save it, Notepad can't. I saved the file elsewhere, but obviously, DB4S couldn't use this file.
I assume this will work on other OS's, that don't lock the files so strongly as Windows.
This was with the latest nightly.