Yes you can add the 5th choice up until the final UCAS deadline, or choose not to use it at all. Would you settle for another course if you didn't get in either because of grades or just lack of offers? Unfortunately both do happen with medecine, or would you be able to do another application round and retake if necessary. Before adding a fifth choice, think very seriously about what you could live with, because if you only get this as an offer would you have pressure on you to take it even if it's not what you want?