What exactly do you need to do, for the purposes of the module I mean? Do you need to just reassign who it is that cultists worship? Does it structurally matter to the modules that there's only 4 gods, or can you branch out a bit? Is it important that all your (for example) Tzeentch cultists in the modules will follow the same demon lord once ported to D&D? Given these are WFRP modules, I'm assuming you don't need Out of the Abyss type stats for when high-level PCs actually FIGHT Khorne or whoever....
Slaanesh could be Malcanthet, or Grazzt, or Socothbenoth. Baphomet is a good match for Khorne, you could make a case for Yeenoghu too (though Nurgle might also have a claim on him). Tzeentch could easily be Demogorgon or Grazzt, or one of the weird obyriths like Pale Night. Juiblex or Zuggtmoy for Nurgle - I'm not sure there's a literal demon lord of sickness, cos in D&D world 'Cure Disease' being available as a level 2 spell makes poor old Nurgle sad.
