Now that Nighthold is out and I've grabbed my 4-set, there are a ton of issues that negatively impact Guardians throughout the raid.
First, and most importantly, the 4-set still has code left over from when it used to increase the duration of Ironfur. If any of you are aware of the Ironfur deletion timers from shifting out and into Bear form quickly while trying to maintain Ironfurs, the 4-set bug is essentially the same bug. An Ironfur with the 4-set will log a deletion timer 2 seconds later than the duration of the buff, causing the Ironfur to delete two stacks: when it expires naturally, and additionally when the deletion time fires off 2 seconds later.
Here is my bug report about the above:
https://us.battle.net/forums/en/wow/topic/20752513242
Here is a video showing it in action. Watch the buff bar in the upper right, the armor value on the character sheet, and/or the purple Ironfur tracker in the center. The stack is deleted at 0:14
https://youtu.be/7pHUjQRJj10
Additionally, fights such as Chronomatic Anomaly, Elisande, and Gul'dan will also cause your Ironfurs to delete two stacks on each expiration as the Ironfur deletion timers are unaffected by Fast Time and similar abilities, while the Ironfur buff is. I have no videos for this.
Here is my bug report for the specific boss of Elisande:
https://us.battle.net/forums/en/wow/topic/20752561565
Both the 4-set bug and individual bugs on Anomaly, Elisande, and Gul'dan pose a lot of issues for Guardian Druids in Nighthold, as you can potentially have your active Ironfur stacks reduced by 1 due to Ironfurs removing two stacks per cast. This can only be solved by causing a massive Ironfur reset by not using the ability for an extended length of time.
I tweeted WarcraftDevs about the issue here:
https://twitter.com/videnfost/status/822348633077116928