Eduardo, many people head north from Nadi to the Yasawa islands. It's beautiful, I hear, but a bit touristy. Take a flight from Nadi to Labasa (on the island of Vanua Levu) if you want to see the "Friendly North." Travel from Labasa isn't ideal, but Savusavu which is close, and Taveuni island I hear are gorgeous. I spent time on Vorovoro with Tribewanted (tribewanted.com) and would recommend that if you're looking for a more cultural and less sight-seeing experience. As for Auckland, there is a good backpacker's scene downtown and around the city - lots of hostels and clubs. I prefer Wellington, however. If you've only got four days, there are some nice hikes around Davenport and Mt. Eden. Bay of islands is nice, but I liked the Kaori forest on the western side of the Northlands better. Have a fun trip!