Bureau number four for me. This one is located in the graveyard of Jorvik, and holds another Codex page and the Hidden Ones Robes
Although it looks complicated, this one is pretty straightforward. Drop down through the breakable planks of the graveyard into the water and then swim through the underwater passage to get to a huge room full of water and suspended platforms.
From here simply keep as far right as possible across the water, and move a couple of blocks to reveal a hole to slide through. Follow the passageway through to the main Bureau room. Here you can find the crypt key on top of the highest bookshelf, giving you access to the adjoining room which has the Hidden Ones Robes and Codex Page
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