What you have done is to change that button so it is now calling your custom command, instead of calling the internal Edit command.
You still see F4 on the button because you kept the Button Label (and it reference the internal command).
But if you want to create a own custom command you need to create a User Defined Command first and then assign a key to that command.
If you then what to same command in a button in the button panel you select so that the button is calling the User Defined Command that you created instead of having the entire custom command there.
But...
A more correct way to do that would be do add notepad++ as editor in
FileTypeSetup.
Then all edit commands in MC will use this.