The fourth and fifth floors of Castle Ember are taken up by the royal suites, vaults, and personal quarters of Queen Corrine. The fourth floor is the one most often seen by higher-level visitors to the castle. Only the Queen, Regent, and immediate family know what's on the fifth floor and they're not about to tell anyone else. Corrine's suites themselves take up the front two thirds of the fourth floor, overlooking the castle courtyard and the Corona?; this affords her a means of looking out over the castle and kingdom without losing some of her privacy in the process.
Most people, if they have personal and private business with the Queen, meet with her in her offices, sitting room, or lounge. Her offices are efficently laid out with bookshelves, filing cabinets, and scroll racks on either side, a mahogany desk in front of the windows opposite the entrance door, and several chairs and a pair of comfortable couches for guests to sit in when they're not being grilled by the Queen. Her sitting room has more seating and bookshelves, and a wall of glass that leads out to an observation balcony overlooking the castle Courtyard. Queen Corrine sometimes makes pronouncements from this location when the Great Hall would not be big enough to hold all the people who should hear it. Her lounge has couches and chairs, several low tables, a window box seat, and a miniature kitchen / bar; she sometimes entertains guests there, and relaxes there when she's not busy running the kingdom.
Other rooms, not as regularly seen, include Corrine's small Library, which is attached to her office (and is adjacent to the top floor of the Royal Library of Ember), and of course the Royal Bedroom suite. There is also a decently sized Royal Dining Room with a small food preparation area and bathroom on the side of the castle that faces the Corona?; Corrine and the Red Knight regularly breakfast there, and sometimes have private dinners and discussion with the family and visitors when not eating down in the Dining Hall.
Queen Corrine's painting studio is also on the fourth floor. This is where Corrine does the majority of her regular and Trump card painting, and keeps extras of her regular decks. A small nursery was allocated near to Corrine's main rooms when Kanna was born; it was later converted back to being a part of Regent Catherine's suite once Kanna was old enough to have her own room on the Third Floor.
Lady Misaki, Queen Corrine's Royal Bodyguard, has a small central suite of rooms on the fourth floor for her personal use, situated so that she is always on-call should she be needed by her Queen. Even bodyguards need to sleep sometime...