Ventilation I: Boyle's Law

Practice: Harry Potter, during the second task of the Triwizard Tournament, is at a depth of 40 meters beneath the surface of the Black Lake where the ambient pressure is about 4 atm. He takes a full breath of air (thanks, Gillyweed!) and fills his lungs to a volume of 6 L. Then, without exhaling any air, he ascends to the surface. What is the volume of the air in his lungs once he gets to the surface? (Hint: This was a breathtakingly—pun intended—stupid thing to do.)