Hi Sandi, I arrived at T 4 this year and did what you are wondering about. I had a few hours between my arrival into Madrid and my next flight up to San Sebastián. I took my backpack with me, and I hopped on the intra-airport bus that's right outside the arrivals area. In about 10 minutes I was at the old terminal, which is broken down into T1, T2, and T3. I remember, I think, that the post office is a flight down. It is easy to find and of the people were very nice. After I mailed my box up to Santiago, I went back upstairs, hopped on the bus, and was back at T4, all in less than an hour. I had to go back through security, which took a little while because it was now the morning rush-hour at the airport, but I still had plenty of time.