Longer version: To the other side of the door from the keypad, you'll see a poster for the upcoming concert. Some words have been circled to create a message. This should indicate to you that you need to talk to someone to get further information.
That someone is the concierge/manager who always talks about the weather. Head to his office which is in the same direction as the ballroom (the door to which is now open, next to the corridor to the elevators at the top of the main staircase).
Head behind the stage via the door on the right and you will find a journal on the desk, and note the circled 4-digit number there. Then return to the door and enter the code.