We were there in October, crowds were fairly low.
We did kiosks at MK, DHS and EPCOT looking for 4th FP+. AK we had no need for one as the only thing we needed or wanted FP+ for was Safari and Everest which we already had and we were okay with only riding each of those once. We never waited more than 5 minutes at the kiosks. The longest wait we had was in Fantasyland. The line looked fairly long when we got in it, but it moved fast and it was only about a 5 minute wait. Other than that, we rarely had more than 2 or 3 groups in front of us. We stacked our FP+ early to attempt to be able to get a 4th FP+ in the parks. The only exception was MK as we had some afternoon times for 7DMT and A&E because that is what we could get, and also did FOF parade.
The very short version:
1. MK- the 4th FP+ and kiosks can be useful. Also, you never know what you might get as people switch things around all day there.
2. DHS- the 4th FP+ was totally worthless
3. EPCOT- the 4th FP+ was only useful because with FP+ where we used to walk on to Figment and Nemo, we found that they now have 30 minute lines for a good part of the day.
The long version with specifics:
1. DHS-- We went to a kiosk around noon to look for a 4th FP+. We were hoping for RNRC as we had used our Tier 1 for TSMM. When we got to the kiosk, they checked to be sure nothing had popped up and we were told there were NO FP+ available for ANY attraction in the park for the rest of the day.
NOTHING- not even the less popular attractions. We were told we could check back and see if something came up. We never bothered.
2. EPCOT --- We checked for 4th FP+ 3 separate days at EPCOT. 2 days we hoped for Test Track as we did Soarin as our Tier one. Day 1- we tried the strategy of using our 1st FP+ (Soarin) and then attempting to move our next 2 FP+ earlier to get to the kiosk. (We had thought maybe we would ride Soarin and then use our 9:30 FP+ if we got there early enough, but we wound up not entering the park until about 5 minutes after opening that day and Soarin already had a 40 minute line by the time we walked there so we rode the land while we waited for the Soarin FP+ to kick in). After riding Soarin, we immediately tried to move the next FP+ up to an early time. The app showed us we were getting a time about 30 minutes earlier- I hit confirm and it glitched and moved our time to 6 hours AFTER our original FP+ time instead of the time it had showed us with no warning.

I could not get our original time back, so I marched off to a kiosk to see if I could get help. They told me it was either a glitch or most likely someone else was trying to get the same FP+ at the same time and hit confirm first and it gave it to them and then defaulted me to the next available time with no warning. There was no availability so they could not get us back to the original time. The best they could do was move it up so it kicked in only 2 hours after our original FP+ would have kicked in. We took it. EPIC FAIL. So, we attempted the 4th FP+ in the afternoon and of course there was no TT, Soarin, or Illuminations available. Our only choices were Mission Space or Spaceship earth for around 8:00 or 8:30 that night. We took Spaceship Earth as we had not ridden it yet. The line was about 15 minutes, so we did save some time with the FP+
Our other day we did not mess with our preset FP+ and we were at the kiosk looking for the 4th by 11:15. No TT, Soarin, or Illuminations available. As those were not available, we did not bother booking a 4th as we planned to spend the rest of the day in WS anyway. The 3rd day we tried we had spent the morning at MK and arrived at EPCOT around 1:00. It was a departure day and ME was picking us up around 4:00 p.m. from YC. After grabbing lunch DD wanted to ride Figment. The line was 30 minutes. I went to a kiosk and got a 4th FP+ for Figment that kicked in 20 minutes later.
3. MK- this is by far where we found the 4th FP+ to be the most useful because it has so many more attractions than the other parks to spread out the system users. This was the case even though we were mostly checking for FP+ later in the day. People switch FP+ around a lot there, so you never know what might suddenly pop up. Day 1 our 7DMT did not kick in until 3:00 p.m. so we were not at a kiosk until about 3:10. This was the day I waited 5 minutes in the Fantasyland kiosk. The park closed at 7:00 for
MNSSHP. Peter Pan was available for 5:00 p.m. We also saw FP+ available for BTMRR, and Splash. Several others were available as well (no SM, A&E or 7DMT). Day 2, our 7DMT FP+ was not until 5:00 p.m. (not an MNSSHP day). I went to a kiosk right after. No headliners available at that time, but there were for some other attractions. I believe we did POTC. After that we went to the kiosk in Adventureland as we were passing near it. If I remember right JC had a 40 minute wait and I picked up a FP+ for about 10 minutes away. I can not recall specifically, but I think we did more than that last because I recall that if we were near a kiosk and did not have a FP+ I would see what we could get. If there was one in the next few minutes for an attraction nearby I would take it. Sure, I got FP+ for things that only had 15 minute lines- but it only took about 3 minutes to grab the FP+ as I walked by, so I would rather do that than stand in the 15 minute line. Seems like I may have grabbed one after JC for Aladdin's carpets. My only complaint about that is that there are not enough kiosk areas. If you have to walk far to get to the kiosk and then far to get back to the ride, that wastes too much time. That is why other than getting the 4th- we would only look for others as we passed the kiosk. Our 3rd MK day was FOF day and we left after to change for MNSSHP. I think I stopped at the town square kiosks as we were leaving to see if I could pull something for between 6 and 7- and I can't even recall if I did. I think it was pretty slim that day- but I might have pulled one for Pooh or Small World. On our departure day, we had an 11:20 FP+ for A&E that we decided to release at about 10:40 to see if we could get something else (a long story- but we wound up getting to see them through FP+ earlier in the week). There were a ton of things available throughout the day- all the mountains were still available). Our choices were limited because we were moving to EPCOT at around noon before leaving on ME, but we took Buzz. The line was 30 minutes when we got over there so it was a good choice.
All in all, we found the kiosks to be fast and the CMs there helpful. They had a ton of CMs there manning them and usually someone at one of them was either totally confused or totally hacked off and they were trying to pacify them. I would not want to be a kiosk CM! Although the kiosks were pretty fast and easy- they are not in enough areas. Also, even though the kiosks were fast and easy, we still absolutely despise FP+.