You should use menu editor (ctrl+e) to edit your menus, just leave 4 (or more) lots empty for the recent files, and make sure you use indexed menus, so that you can catch the menuclicks in the same event.
To remove the slots, just put their property to invisible.
Use the registry to store the filenames, (look up savestring and getstring for simple use) and move down the filenames to the next slot in a for next loop, when you add the next filename. To check if the recent filename has been used, just compare them in a for next loop.