I ran the previous adventure last week and it went almost 8 hours! So I had an eagle eye out for places to trim and speed things along in this module. As others have said, this adventure feels like a fitting and exciting conclusion to the trilogy and there was great tension throughout. However, this module is, once again, too crammed with battle to in any way, shape, or form be run in 4 hours.
To begin, I knew my players have a bad habit of really agonizing over "choose your adventure" steps in modules. I, personally, love that there's an option and think it makes the adventure much more realistic. However, I knew that would bog down the game, so I just hand-picked which steps the PCs would handle according to what I think they would get the most out of. That worked out extremely well, I think, as all of it still felt organic and our big debater was like "Yes, Aliana! That's a great idea! Good luck!" So they still felt empowered, hahah!
I did end up, though, cutting one of the battles because A) the module was too crammed, and B) it was kinda boring :/ We skipped the Otyugh battle because it didn't have much effect on the story beyond "Oh shoot! All kinds of crap is happening!" which I think was accomplished elsewhere. Moreover, if the purpose of it is to expend resources, my PCs were pretty spooked early on when bodies started popping back up as Vine and Needle Blights, so they used a good bit in those previous fights, hahah! Three big beasties just before the PCs fight three more beasties (Strong party) and a caster, that would have gotten old. So we just skipped it and I think the adventure was better for it.
What I think this module does best is how it mucks with players' metagaming just a bit. First, there's no long rest! Our 9th level Bard blasted all of his top level spells in the first encounter and was like "Okay! Time for a long rest, right? That was a big fight." No sir! So that was a fun to watch him realize the consequences of trying to read the flow of the module. Also, those cultists! When they died and I just left them on the battlefield, my players were like "Do we need to take them off the battle map?". Nah, let's leave them! Adds to the chaos! "Oh cool!" And then I twirl my mustache and wait. Suddenly, BOOM! They turn into monsters and attack! My PCs were super freaked out and hyper suspicious of anything else they killed for the rest of the night. Which was awesome when what they killed didn't turn into something else. Hahah! Definitely made for some memorable scenes and tension.
All in all, this module and trilogy is really good and I had a blast DMing them. The extras that the creators added in here throughout the module are exceptional and do much for the DM's and PC's quality of life in terms of running the adventure. But I wish the adventures were just trimmed down a bit, especially for bigger battles. Both this one and the last one benefited from cutting one of them and there was no consequence in terms of lost story or atmosphere. And even with cutting the third battle in this module (because there are 4!!), we still were at 6 hours. Not awful, but still long, hahah! So my advice for DMs is to be aware of what can go and don't let the module get bogged down--it's too good and deserves everyone's full attention! :)
(PS - some bad typos in this adventure too, which is rare for BMG modules. Definitely could have used another round of edits.)