Thanks for your help

Yes, the 4 hour layover is very short. My trip is from NY to Hong Kong (March 8-9th) but there is a stopover in NRT. I actually wanted to stay in Tokyo for at least a day but the increase in ticket prices (about $350 difference just to stay for 1 day) didn't justify it.

I have to bring over 2 computers for my friend which are heavy and a bit costly - not good for anyone else to handle. Besides, I know no one else to help me there.

I was also thinking that if I "miss" my flight, I can go to the counter and tell them to book me on the flight the following day for no extra charge. I've done that before in Hong Kong and there was no charge to do so. I would chance it there in Narita so that I can stay long enough to go to downtown, get some good sushi and continue on my trip the next day.