It's possible to use four year's worth of points, but your trip would have to fall over your UY and you would probably have to make two reservations and link them.
For example:
You have a Sept UY. You plan a trip for late Aug through early Sept. You could use banked current and borrowed points for the days through Aug 31. Then on Sept 1 you could use borrowed points from the next UY.
Like a trip from Aug 15, 2009 to Sept 15, 2009:
For the Aug 15-31 nights:
Use UY 07 banked points (good from Sep 1, 2008 to Aug 31, 2009),
UY 08 current year points (good from Sept 1, 2008 to Aug 31, 2009),
UY 09 borrowed points (good from Sept 1, 2008 to Aug 31, 2009),
For the Sep 1-15 nights:
UY09 current year points (good from Sept 1, 2009 to Aug 31, 2010),
UY10 borrowed points (good from Sept 1, 2009 to Aug 31, 2010).