The Seven's Sawmill

This series of encounters can be run in a largely linear fashion, thanks to the floor-by-floor design of the sawmill. There is one significant difference between this set of encounters in 4th Edition and in the original adventure: each encounter should be isolated, so that (barring extreme circumstances, like a PC running madly up the stairs into every workroom) the party will only be dealing with one encounter at a time. This means that the cultists are confident enough of their skill and numbers that they can handle the group, and that other floors' cultists will not be alerted to the intrusion until the party reaches their floor.

The reasoning behind this is simple. If even two of the encounters in the sawmill were to combine, the players would have an extremely difficult fight on their hands. To illustrate this in mechanical terms, imagine that floors 3 and 4 each have 1,500 xp worth of monsters in their respective encounters. Individually, these are level 7 encounters and completely appropriate for the party at this point in the adventure path. Combined, however, they total 3,000 xp worth of monsters - a level 11 encounter, at the upper edge of what the party might be able to handle (and possibly resulting in PC deaths). And if three such encounters were combined for whatever reason, the party would have a level 14 or 15 fight on their hand, far out of their league.

This wasn't a problem in 3rd Edition because encounter design was much looser. The ease of destroying large groups of monsters (a well-placed fireball, for instance) made increasing the size of an encounter a non-linear xp progression. In 4th Edition a greater emphasis is placed on non-minion monsters being credible threats for a number of rounds in order to create dynamic, tactically interesting fights. It also means that the party really can be overwhelmed if too many non-minion monsters join the fight.

It isn't difficult to justify this isolation even within the same building. The undermill is completely cut off, internally, from the other floors, and the machinery there obscures the sound of combat. The sound of machinery and other workshop noises permeates much of the rest of the sawmill as long as the waterwheels stay active, which makes hearing anything outside one's own room a feat.

That said, if you really want to threaten your players with the possibility of alerting larger portions of the sawmill, make any additional monsters that join the fray from other floors into minions (use Snaketongue Initiates). Keep the encounters on the other floors intact - they will play out as normal once the party actually reaches those floors. The cultists are simply sending their newest initiates out as reinforcements, or to slow the party down while they prepare their own defense. If this does occur, play the cultists on the alerted floors as being aware of the party. They will have already donned robes and masks by the time the PCs arrive.

The sawmill contains a total of five encounters, all of which are at or above the party's level. They should have a lot of incentive to fight all the way to the top of the mill without taking an extended rest, lest the cultists go into hiding or somehow procure reinforcements. If the party does chicken out and isn't able to finish the mill in one go, you'll have to adjudicate how the cultists deal with losing a significant number of their members (as well as the knowledge that they're exposed).

Note that all of the cultists' melee attacks with the Weapon keyword are made with a war razor, regardless of what the suggested stat block calls the attack.