New Series

[phansen]

NFHS
Rainy night and the coach asks to change out the game ball. I said you can't until the series has ended. They proceed to get a first down and the coach asks me again to change the ball. I tell him again that you can't change the ball until your series has ended.

Does a series of downs mean an entire drive until they relinquish the ball, or until they get a first down? (Maximum of 4 downs) I read casebook 1.3.2 and my exact situation doesn't apply, but from the other situations explained I'd say I was in error.