Interesting. Are there particular threads where people report on this? I just use my experience to determine whether it is even worth my time to try for a fourth FP (not sure what other practical purpose the knowledge might serve) but that may be fairly irrelevant now that we can make our fourth FP from the app.
Every potential aspect of FP+ was discussed and possible angles and procedures postulated... long before they even released the first FP+.
After FP+ came on-line, there were hundreds of posts questioning every single aspect, down to the smallest details.
Months and months of daily FP+ Q&A and reports on what happened on X date, before during and after WDW trips.
Whether X kind of ticket or AP would be better, or offer a different FP+ choice and experience.
Whether booking in advance by 1 day, 5 days, 30 days, 45 days, 60 days, 60 days, plus length of stay... or day-of booking, and at what time, day-of... would make a difference as to availability.
Whether booking as a single guest or 2 guests, or a dozen guests, would make a difference.
Does booking for a second park (for park hopping) yield more choices.
Does making CHANGES in attraction FP+ the day-of, or canceling vs. letting FP reservations expire work better?
Attempting to book another FP+ after scanning at the first Mickey Head, but before scanning at the second Mickey Head.
Would booking FP+ starting at day 10 of a 10-day trip yield substantially better choices?
You get the drift.
These things have come under the DIS microscope daily over the past 2-3 years.
There is a LOT of anecdotal evidence.