Bought used Ipod, how to get my music onto it?

I have an Ipod nano 5th generation that recently broke, so today my Dad and I got on ebay and bought an Ipod nano 4th generation.
I don’t know for sure if the memory is cleared on the one I just ordered, but I sure as hell hope so. I am thinking I will sell my 5th generation for parts or repair when the new one gets here. So I have three questions. Please answer all in complete detail if you can:
1. How do I clear the memory of the 4th generation if it isn’t cleared so I can put my music on it, and if it is cleared how do I make my Itunes understand that that is my new Ipod and add my music to it?
2. How do I delete the 5th generation from my itunes and make it so I can add the fourth generation?
3. After I got my 5th generation for Christmas 2010 I bought a 100 dollar speaker for it, will my 4th generation work with that speaker?

I know it’s confusing, but please try to help.