Both girls actually have a scene on the 4th floor, but they are mutually exclusive: whichever girl you found first will be seeking, the other one will get a scene.
For Kanako (the girl with the pigtails): in the corridor at the top there is a room with two doors inside on its left wall. The top one will be locked, interact with it before going to the door below. In the room there will be some papers with a glowing spot on it, which is the key you need. However, you will need a second item as well, which is in one of the patient rooms in the middle of the bottom hallway, on the bedside table of the middle bed on the right side (another glowing spot). After you get both you can enter the locked door.
For Rie (the short-haired girl): interact with the wooden door in the right side corridor, it will also be locked. Go straight up and into the first door in the top corridor, then go to the printers in the bottom right where there will be a glowing spot. After you picked up the item there you can enter the locked door, the girl is hiding in the locker in the top left.
Important to note: if you go up to the roof during the 4th floor search, you will find some graffiti that you can interact with, which will show a scene of a man going up to you before running away. If you don't do that BEFORE seeing the scene with the girl that is hiding, then after that scene the other girl (the seeker) will be missing and start a chain of NTR scenes (which should probably be added as a tag) leading down to the basement. If you DO go to the roof, it will instead go straight to an ending, skipping the scenes with the other guy (so if you don't mind that sort of content, I would recommend not going to the roof, as you'll get more scenes).
On a side note, the NTR scenes are pretty lazy: they play out almost exactly the same (even in regards to dialogue), and they also simply reuse the images from the regular vanilla scenes on the 4th floor (which you wouldn't know the first time around, as they are mutually exclusive).