Can you get into the phone at all to confirm what the baseband is, or can you ask the person you got it from? (The iPad BB is 6.15.) But assuming it is, you'd download Pwnage (Mac) or Sn0wbreeze (Windows) and the 4.3.3 ipsw. These programs let you customize your firmware before you restore, which gets around a lot of problems with iTunes. You load up the ipsw, check off the options you need, and it rewrites the firmware just for you. Then, you restore to that firmware in iTunes. Ipheuria has written a lot about this, just search the forums for his name and the relevant tool.