Q: month end relative recurring event

I'd like to create a recurring monthly event that is relative to the end of the month; four days before the end of each month as a reminder.

My following lame attempt didn't work:

I thought it would be as easy as editing the plist for the event, but I'm too naive about how this works. I opened the last-day-of-the-month event plist file I found at "/Users/<user>/Library/Calendars/Pending Events/<blahblah>.plist and changed the string <FREQ=MONTHLY;INTERVAL=1;BYDAY=SU,MO,TU,WE,TH,FR,SA;BYSETPOS=-1> to <… BYSETPOS=-4>, but that had no effect on the event when relaunching iCal. I didn't find any other mention of my last-day-of-the-month event in the path, so I assume this is the critical file. No idea how it reloads, which might be the problem.

Obviously, I could plow through this manually creating events but that's no fun. Is it possible to build this kind of recurrence?