There once was a web enhancement for this on the Wizard's site called "Tougher Sons" or somesuch. I used that, plus did some heavy reworking to fit it into my Scarred Lands game. The party was around 9th level, and had 6 members, and instead of a hostage rescue it was a "grab the McGauffin" mission. Still, even though they bypassed the entire upper works, they got trapped below when the fighting started, but at higher levels, both the PCs and their opposistion have a lot more options (like Teleporting away once the whole orc horde is after them).
Had I not so heavily changed it, I could probably answer your questions better (I completely redid the dungeon part, and massively expanded it overall). But at 4th level, the party should have access to alter self, invisibility, silence, etc. Getting in shouldn't be too hard if they scout the place out propperly. Getting out is another story, however.
