Grand Hotel 4th floor
You are in Room 408 but you can't go directly to the corridor. Go through the broken wall instead to get Room 406. . There is a letter on the shelf.
A swarm will attack you when you reach the corridor. Remember to turn off the flashlight - there is a nurse nearby. When she comes out - wait in the dark and then quietly knock her out.
You can look through the hole near Room 403.
This the only room that you can now enter. Push the wardrobe to open a way to Room 404.
Go to the bathroom and get the Alchemilla Postcard from the cabinet. That's one of the three items that the disembodied voice from behind the door wanted.
Go to Room 408 and after a short scene with your old friend get ready to meet a Swarm.
You can go back to Room 301, give back the postcard and save the game. In any case you will need to go to Room 405.
There is nothing important here, except for the hole leading to the next room.
Leave this room and go to the corridor.
Go to the room 408 and hack the boards near the door. Go through the hole, destroy a Swarm and take the briefcase with serum #2.
In the north-west part of the corridor you will find another entrance blocked with boards. Chop them, jump over the crack and go upstairs to the 5h Floor.