You must remain below FL45, or below 3500ft, which ever is higher. If the local QNH is > 1013, than you can fly above 4500 ft altititude, as long as you remain below FL45. IF you have 2 altimeters in your aircraft, set one to the QNH, to give your altitude, and the other to 1013, and ensure that you do not climb above 4500ft on the altimeter set to 1013.