Prepaid SIM Card if will cost you $20 and comes with $20 of prepaid credits. Where you guys it they may set it up for you at no cost to you or you can change your SIM Card yourself to the new One through your self serve account which is also no charge. You can call and have customer service do the switch for you however there is a charge if you call and have them do it.
It will cost $10 for a nano sim (if you're on prepaid, it'll cost $20)
If you want to transfer your contacts that are on your N4 SIM, you should synchronize your contacts with Google on your N4, then synch your iPhone with Google using CardDAV.