Everything around there starts about 5 am. The trains start their day from their maintenance base at Northfields, a few stops east, so the first ones run west from there to Heathrow, round the loop or out to T5, and then start back towards London.

There's only one significant way in from the terminal to the T4 station, there are ticket gates (and baggage restrictors) across the entrance, so get friend to wait there. There's also an Underground official at the gates if either of you feel like chatting. Because the trains wait at T4 for several minutes you can get off, say Hi to friend, have them come through the gates, and get back on the same train. If you don't go out through the gates and in again you won't have to pay an extra fare.