I had the same question on behalf of a friend last July. His flight was at 6:30am and he ended up reserving and taking the Aerocity shuttle at 4:30am from Calle De La Cruz to T-3 for 23 euros.
I would have no problem taking the N-4 and finding my way around in Barajas town but a first timer in Spain is bound to miss the flight.
The way from N4's last stop to Airport T3 is about 10-15 minutes walking, but it includes crossing an open ground. That is something I would definitely not advice to anyone:
Registered: 04/04/07
Posts: 6
Loc: Salisbury, England
Thanx for posting about the Metro ... I wondered about this as I'm using T4 for the first time next week. Hope it's not so far to walk as from the other terminals