Originally Posted by Hhonor Gguard

OK, here's the inside scoop straight from an internal communication:

The Q1 Promo runs from 1 Jan to 31 Mar. EVERY 4 stays or EVERY 10 nights will earn you the free night certificate to any Hilton property.

All properties worldwide are automatically opted in as participants both for earning and redeeming. We must submit a waiver and argue a case for opt-out.

I do not see any fine print regarding an earning cap or restricted properties.