If you're a Disney resort guest you can also make the 4th FP at any park with your resort concierge. We took advantage of this several times on our recent trip because the FP kiosks are in less than convenient locations sometimes.
We were there 4th of July week so it was super busy and I'm sure this has an impact, but the headliners were gone when we went to book our 4th FP's around 1 PM for both parks.