The FAQ is generally OK on this but as with most things - not always that simple. I've never had a problem getting a 4th FP for almost any ride at the parks. However, there are definitely things that affect that:
1. When are you going? Much easier during non-peak times.
2. How many of you? Much easier to get a 4th FP for 2 than for a group of 8.
3. How much time do you want to spend on your phone? This is the key one to me. When you check for 4th FP availability on your app, the availability with change each time you refresh. Availability can literally change every few seconds. Spending 10 minutes while you're walking or online continually refreshing will often bring up some rare FP openings. Again - crowd size and party size will affect this. And usually when one of those rare ones pop up it will be for 1-2 times slots and not wide open. But last trip I got Soarin (multiple times), 7DMT, RnR, TSM as 4th FP. Doesn't always work - you're at the whim of timing and it will likely take more than 1-2 checks. But don't give up if there's something you really really want (and although I haven't been there since opening, I wouldn't count on this for any of the Pandora rides!)