One of the better tier 4 modules I've run. The fact that Dagon is the boss is a fantastic addition, and he gave my players a run for their money even at their levels, so that's always good at higher level play.
If the players know or figure out how to bypass most of the encounters, they'll clear it in well under the four hour timeframe; my players got it done in two and a half at most, thanks to plenty of Greater Restorations and the occasional bribe. Though I did find that the encounters that are easiest to bypass are also the ones they'll bother fighting because they can clean them up so quickly.
I mean, we have adventurers who can and have stabbed gods in the face, a few Nalfeshnee mean nothing to them.
But the saves and skill checks were high enough difficulty to give them some trouble, and the stacking exhaustions were pretty nasty. The final battle also proved fittingly epic, with Ubtao granting them a magic item to battle against Dagon.
Fantastic choice for a magic item, by the way. I made sure to print out a high quality cert for my players just because it's that worthwhile.
Overall, good challenges, good mission, great magic item and final boss; the only thing keeping this from five stars is the lack of threat from the 3/5 of the demonic forces to encourage players to think smart about bypassing them. But this is definitely a worthwhile t4.