“A man may do both. For not we but those who come after will make the legends of our time. The green earth, say you? That is a mighty matter of legend, though you tread it under the light of day!” —J.R.R. Tolkien
Sort of: three of the PCs will have adventured in Thieves Fortress Badobaskor prior to taking up the long trek to Tsojcanth. All of the PCs, though, are our "B-team" PCs, so they're starting at 6th level (our primary/A-team PCs are all 9th-12th level).
Are you running S4 more or less as published, or doing anything unique with it?
Depends on how things go, but my intent is to add the Underground Lake level for sure, and perhaps another level or two as well (a level with Iggwilv's labs, conjration rooms, living quarters, study, etc.; another that's guardrooms, barracks, empty vaults, etc.). Some of the players were pretty against Caverns of Thracia when we started this campaign 3 years ago, so I want to be sure that we don't end up in too-similar a situation with LCoT. So, I think the only one that's a definite addition is the underground lake.
I'm am going to drop an NPC into my game that may steer the party to this adventure. The party's in the right level range and this has always been a favorite of mine. In Rappan Athuk there is a cursed warrior in the dungeon, Marthek in the published version, but he was killed by other PCs a while back. I've replaced him with Saringar, a knight errant from Bissel. Cursed by the priests of Orcus, he now serves as a guardian on the 2nd dungeon level. If they rescue him he will try to get them to come with him back to Bissel, where they will be drawn into the Tsojcanth plot.