While I didn't get FP+ for Wishes as the 4th FP I did get them for Illuminations in Epcot and wanted to share my experience.
We are a party of 5. When looking for a 4th FP+ on the Kiosk, Illuminations was not available. So I took what was, which was Journey into the Imagination with Figment.
Then between that time and our FP time for Journey, I looked for other options. I broken up our party and selected 3 people and searched. Illuminations was an option so I selected it.
I then tried looking for the other 2, Illuminations wasn't an option for a party of 2.
I then split the last 2 up again, selecting only 1 to change and Illuminations was again an option so I snagged it.
And finally I searched to change the last person's FP+ and Illuminations was once again an option.
So by splitting my party up I was able to get us all FP+ for Illuminations, and this was probably around 5-6pm which is pretty close to when the show actually starts.
Why there wasn't a party of 5 available in the first place, or even a party of 2 for the last 2 individuals available I will never understand. Unless I just happened to get lucky and 2 separate individuals cancelled in the amount of time I was riding the boat ride in Mexico...
So If FP+ is still available for Wishes when you go then you might try that. Or even try it for any other 4th FP+.