No, it isn't possible. You can bank 1 year and borrow 1 year for a total of 2 years in addition to the current UY. If your trip falls over end of 1 UY and into the beginning of the next UY, you can borrow points from the next UY after that, too, but I think only for the days that fall in the new UY.
That's confusing - here, let's do it this way. Say your UY begins Feb 1 and you have a trip booked from Jan 25 - Feb 5 2012. Here's how you can use points:
Jan 25-31 2012: You can use 2010 banked points, 2011 current points (you'd still be on 2011 points because your UY is Feb) and 2012 borrowed points
Feb 1-5: You can use 2011 banked points, 2012 current points (remember, you're in your new UY now), and 2013 borrowed points.
That is the only way 4 UYs of points can be involved in 1 reservation. At least, that's how I think it works.