Upgrading to an iPhone 5

Hello.  I've just bought an iPhone 5 and restored the backup from my Iphone 4 on itunes.  iTunes now thinks both phones belong to my mobile number and calls them both Simon's iPhone.  I would like to make iTunes forget the iPhone 4 as being me so that my wife can use her identity on that phone.  How can I tell iTunes to forget the iPhone 4?  I would then like it to recognise it as belonging to my wife rather than me.

Thanks for your help.

Simon