I too understood you had 90 days from the day you signed up for it.

I believe they didn't post the full terms was they wanted people to sign up and then do the four stays in the next 90 days, making me for example add a stay or two to make it happen if I was borderline. If you knew the full set of rules, you would just sign up when the four in the 90 day timeframe worked best for you, and they wouldn't get any extra stays out of you. I know that for myself, I have changed two non Hilton planned stays to them to get the four in.