I may have answered my own question:
"If a member completed 75 or more Eligible Nights in 2011, when will they start earning 4 Starpoints per dollar?
SPG members who completed 75 or more Eligible Nights in 2011 will start earning 4 Starpoints per dollar on March 1, 2012 and will continue to earn 4 Starpoints at least until February 28, 2013."
So, you do not get this benefit for the whole next year. That makes it less attractive than I thought. Few people can earn 75 nights except by ~1/2 the current year in, so the benefit is roughly for 1/2 a year for most people...
I believe the Feb 28, 13 date would be if the person failed to get 75 nights during 2012. The March 1, 2012 date is due to the fact that the new benefits (including 4th point) were not rolled out until then.
So you met 75 nights in 2011, earned 4th point all throughout 2012, and if you didn't get 75 nights in 2012 the 4th point would stop Feb 28, 2013. So you've enjoyed the 4th point for the entire 2012 year and 2 months in 2013.
I think the 4th point benefit follows the same timeline as earning platinum and all other associated benefits that carry over the following year and then 2 months into the next year.