What you are trying to do is not how it works. You must make one reservation encompassing five nights. It will not help you to have four nights reserved and wait for the fifth night to become available.

From the Hilton Honors terms and conditions -

5th Night Free: For every Standard Room Reward stay of five or more nights, every 5th night is free. Members can utilize the 5th Night Free benefit on an unlimited number of stays annually. Applies only to five consecutive nights within the same full Points Standard Room Reward stay at the same property, up to 20 nights, when confirmed at time of booking.