I'm in the shadow vale, and I gather I need 4 keys to get into the final crypt, and I can't find the last one. I've been through all caves several times, but all I come up with is 3 keys. The bottom right hand cave keeps re-spawning skeletons so I'm thinking this is the one where the final key is hidden, but still, no luck. I've opened all coffins, boxes, statues, skeletons, etc. Are there any hidden doors in any of those crypts that I'm not finding? How would I find them? Would the Find Traps skill work?

Well, you need the Gate key to get into the crypt. the rest of the keys are inside.
Are you trying to open that big hidden door in the room on the 2nd floor where a skeletal mage in the center crypt type area? look for a switch