Because the Sun is low you can take photos all day long to traffic going in/out of T4, which is mainly composed of Iberia and Air Nostrum. You will miss however all the Iberia long haul has they taxi to the T4 satellite terminal. During the afternoon, I've pointed my lens to the East and took photos of traffic going in/out of T1/2/3, so, more variety.
Light was perfect all day and I've done spotting from sunrise until sunset. In the end I've packed my gear, treat myself with a great meal and took the bus to T4. If you take the subway you'll have to pay an extra fee (3€!) and the bus ticket is way less than that and you can buy it onboard the bus, from the driver. I took the overnight bus back to Lisbon thinking how great the day was. I've been many times to Madrid airport and it never fails to surprise you.
