Technical question about FP+

[andypooh]

Are you eligible to book a 4th FP in the park as soon as you use your 3rd FP or do you have to wait for the window of that last FP to come to an end even if you are already done with the ride? Thanks.

[DizneyRox]

Reports I've read seem to indicate when it's used. It's not used until the second FP check in the line.