I can confirm the 4th FP. For me, it was back in August 2013, when they had the first "real" test of the whole system, but while legacy FP was still around. I wasn't using the app to book (because it was even buggier then), but, on the website, you would choose three rides like usual, and then when you got to the final page, you were assigned a time for a FP at a specific ride (kind of like the old surprise FP- for Philharmagic, etc.). You could then change the attraction or the time to any other ride.
It only happened at MK, and I always got rides like POTC, HM, and JC. Of course, at the time, only four resorts were testing, so availability was pretty much unlimited, and those rides didn't have the waits that they do now.