There is no secret door there, once you kill the wraith in area #4, return to the first room and continue to the rest of the dungeon, you will be able to access the rooms near area 4 from another place, don't worry 
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"When I am working on a problem I never think about beauty. I only think about how to solve the problem. But when I have finished, if the solution is not beautiful, I know it is wrong."
Buckminster Fuller | 
Linear Mode
