I'm going as well. Have booked the thing and am arriving early. Already on the 2nd in the nearest town. From what I have gathered the gates open officially on the 4th, but if you booked the Boom bus you can get in on the 3rd, but by a different gate.
So I was checking possibilites. No boom bus for me, too late for that... But I would like to arrive on the 3rd if possible by a little. If I need to wait till the 4th, I will try to be as early as possible. Seems to be a massive inflow of people, so that's going to be tricky. Anyway if anyone knows a way to circumvent this issue please let me know.
Btw they write that busses will go from the nearest town (Castelo Branco) to the gates on the 4th in sync with trains and busses but they don't specify times. Will send them a mail though to check out how early they will have busses going...