Rule 5, Section 1, Article 1

The team which next puts the ball in play by scrimmage following a free kick, touchback or fair catch is awarded a series of four consecutively numbered downs in which to advance the ball to the line to gain.

a. Each awarded first down starts a new series of four consecutively numbered downs.

b. The referee shall correct the number of the next down prior to the ball becoming live after a new series of downs is awarded and prior to the declaration of the end of any period.