I said "most likely". I read every post on this thread and others for almost two months now, almost every time someone mentions this happening on an older device it's an issue that either started before 10.11.4 or just beachballs and hangs for ~30 seconds that always recover.
There are way more mac computers in circulation than the latest 2015 versions, yet, 65%-75% of users voted that this issue affects their 2015 machines which is the bug discussed in this thread and the reason I created the poll.
You gonna ignore the 25-35% of the non 2015 Macs and tell them they "most likely" have another issue even though i´ve seen alot of non 2015 Mac users have the very same freezing issues, what a weird stance. Don't ignore people on older hardware just because a very small percentage of users misunderstood the issue and reported beachballing instead of complete system freezes, those beachballing issue have been around since before Mavericks. Also, the percentages fits pretty good with the sales statistics so percentage wise the freeze issue affect an even range of Macs, also, people with older hardware maybe doesn't bother updating to newer OS X release as fast as people with newer hardware, there is alot to consider before ignoring non 2015 people that have this issue.
Also, there is no evidence this is an hardware issue at all, could just as easily be a pure software issue.
Your issue also seems to be different although you have a 2015 iMac:
"So, so far 10.11.5 PB3 and PB4 have not yet been freezing for me, which is great, the problem is that with PB3 it introduced a new bug for me that is still present in PB4. My monitor on my iMac turns of from time to time, and it is impossible to turn it on again AND this only happens while using Safari, when using any other of my 10-15 programs i usually use, those have never invoked a black screen".
No, i don't have a 2015 iMac and my issue was the very same freezing BEFORE PB3 and PB4, PB3 and PB4 fixed the freezing bug but introduced a NEW bug for me, the keyword is "NEW", before PB3 and PB4 i reported the very same freezing as everyone else, the reason i wrote about the new bug, is, as a stated in that very post that the new bug happened EXACTLY at the same points that my previous freeze issue did (scrolling in safari after about 30 minutes of usage)
I also said earlier if apple engineers focus their efforts on one hardware that is mostly affected "rMBP 13 2015" the issue should be solved for everyone if it's the same bug. It is easier to debug and pinpoint the issue if they test it on one hardware than look at a dozen similar issues.
Actually, it is the other way around, if the bug is reported on several different hardwares the developers can more easily find a common denominator and deduce the problem area, developers does not limit their debugging to a single item but rather a system or sample size as a whole. If they blindly just focus on one hardware they miss the common denominator that could give them a clue about the origin of the issue. And why should they focus on one type of hardware if it turns out it was a software issue instead? Then they would miss the bug completely.
I´ve seen so many speculation in this and other threads, some claim it to be a graphic driver issue (they argue the same way you do, "lets just focus on my hardware and ignore everyone else on completely different hardware with the same issue"), some claim it to be a Broadwell issue with the very same argument.
The closest speculation is an issue with the "WindowServer", Safari have had problems with WindowServer before on other releases of OS X, most had problems with Safari causing the freezes and some others on other apps, all of witch the common denominator is WindowServer, if WindowServer crashes without being able to restart itself OS X would experience the very same symptoms that we all have reported on. If WindowServer did broke in 10.11.4 and Apple fixed it on 10.11.5 PB4 and the upcoming GA, the freezes will be fixed for everyone no matter what hardware they have experienced the issue on.