There are five keys and two other objects you need in there. The two others are the Symbol of Myrkul, which is used before this, and the Black Wolf Talisman, which is what you need now.

This item is in a container in a room to the south. Lots of people who don't search well miss it. In order to get into that room, you need a key from the symmetric room in the northwest, which many people also don't find. Apparently you don't have it, because the four keys you do have are all used before this point. Here's a link to a map which shows this whole level. You should be able to see what you're missing.

http://www.tgeweb.com/ironworks/icewind/maps/vale06.jpg

I can't pull the switch in the 5th crypt. It says it is locked even though I have gone through all the other crypts up to this point. I have 4 keys, how many am I supposed to have at this point? The gate earlier in this crypt did the same thing. I left and came back and it finally opened, but the switch is staying locked.