I kinda like the "one entrance" lounge reception on the fourth floor, left side to Sakura and right side to FC lounge. The appearance of the lounge and the control for the opening hours are more uniform that way.

The fourth floor FC lounge used to be accessible via an elevator next to the FC check-in counter on the third floor, but I think a lot of people didn't even realize there was a fourth floor FC area. I have often seen only a handful of people upstairs before the renovation of the fourth floor into two different lounges. It was great for me before, cuz it was nice and quiet, but I have often sat there thinking "what a waste of (food & human) resources."

My guess as to why there are two entrances to the 3rd & 4th floor is that they can better control capacity and staff usage for their busy/off-peak hours. The one entrance also clearly tells people when the upstairs lounges are open/close, unlike the old elevator access to the FC, where a sign was just stuck in front of the elevator saying that it was close for the day (after morning flights).