To be honest, if you are travelling First class on Christmas day, there will be no traffic whatsoever at that time of night, so I would book a car with Addison Lee and aim to arrive at T4 as close to the flight closing time as possible. You will find it a creepy, spooky, haunted ghost town. Drink copious amounts of champagne upon arrival at Gate 1.
Sage advice. T4 is just spooky, the lounge crap and overcrowded. Get there 1h15 before hand, spend 30mins tracking the champagne down and then be grateful to get on the plane.