Do you have to scan through your scheduled FPS in order to make your 4th,or just let the time of the 3rd expire? I have a throwaway 3rd and don't want to scan if I don't have to.
If you tap the FP tapstile and then walk away, you'll be able to immediately book your 4th FP. You don't have to wait until the full hour is up. Tapping #3 is faster than waiting for the window to expire.