My question is this - can I just pay the 3100 yen ticket, hand the package to him without going out of the station and hop right back on WITHOUT getting a "return ticket" back to NRT?
No, as mentioned the Narita Express and Keisei Skyliner, the two fast train options you have are all reserved seating, so you would need two tickets.
But thats beside the point, 4 hours is cutting it awfully close, ksandness is right you never know what might hold you up, Immigration and Customs can be up to one hour, then you have to get currency exchanged to purchase your ticket if not credit card/ then you have to wait for the next Narita Express or Skyliner to pull up, then it takes 1hr just to get into Tokyo, arrange to meet your friend, then wait for the next Narita Express or Skyliner to depart another 1 hr, after arrival back at Narita Airport (in terms of Terminal 1, you have to clear about three different security checks, and you have to go through exit immigration as well before making it to your gate before the door closes.
I agree with having either you ship/mail it to your friend in Tokyo, or have your friend go out to Narita Airport and pick it up from you.