Chapter 2 Missing Letter

[MmmBrains]

I'm sort of a perfectionist and I am having trouble finding the last letter in chapter 2.

- Letter received after talking to tortured soldier near the start of the level
- Letter found in first secret room, on the table
- Letter found in third secret room (water room with lots of explosives), right after you get the shotgun
- Letter found in the last area where you fight the spirit for the final time

I already played it through twice and I still can't find it