From Mythic+ testing on the PTR, players have discovered some large undocumented Mythic+ changes for the Season 4 dungeons.
Thanks to Tarithal, Dorki, Yoda, Kyrasis, Xepheris and Arty for helping compile these changes.
Documented Changes
Some good changes to Algeth'ar Academy have been documented by the community.
A small but lovely change for the final boss:
Some undocumented and big buffs to the dungeon for Brackenhide.
Melee players will have to watch out for Wilted Oaks now as you'll have to dodge the very hard to see frontal. Ranged can simply outrange the breath by standing 30+ yards away. Then as if Rotsingers already weren't hard enough, they're super banned now in S4, although it's likely that the Totem double dipping is a bug.
The Beastcaller might be the most inefficient mob in the entire dungeon now and likely will be avoided. The new Desecrated Ohunas also make this section a pain as you have to