As others have said, take 2 hours out just for the journey to Shibuya and back (1 hour each way) on the NEX.

Also, take 2 more hours by the time you get out of your plane and go through immigration, customs, figure out your NEX tickets and sit on the train (which runs every 20-30 minutes), and the for the the security clearance, immigration etc. on your way back.

That gives you 2 hours. Should you do it? Yea sure. If you feel adventurous. Shibuya is a good station to go to. The exit is right at that crazy intersection, but I don't think it is nearly as mad as the Tokyo station (which is a humongous labyrinth). You can find lots of eateries nearby, sit at the Starbucks, as mentioned, and just watch the mass human crossings; and say hello to Hachiko's statue as well.