Each phone can be set up separately.
If you are giving away your device, see what to do before selling or giving away your iPhone, iPad, or iPod touch
Also check this article again:
Texts are not stored on the SIM. You should set up the phone as new device to get rid of all personal info.
Thanks for that. So Now I was wondering - if I reset the whole iphone 4s as stated above, does the new owner need a computer to create an itunes account? Or can it be done on the iphone? She doesnt have a computer/internet, but I figured once she has the fone connected with data or goes to public wifi, can she just create an account by using the iphone? Then she can install her own apps etc. And then in a few mths time, can I plug "her" iphone into my itunes and put some of my music on it? Or can she only use her own itunes account? Sorry to be a pain. Thanks again
An iTunes account can also be created on an iDevice, check this article:
You can only sync to one computer at a time. Every time you switch the computer and try to syc, iTunes will erase the phone and replace the content with the one from the iTunes library on the computer.
And you can only play bought media from iTunes on devices that are authorized with the same iTunes account.
You can sync music you imported from CD's.
In your case it would be best to create a new user account in Windows and set up iTunes with her account info. That way no media and other data can be mixed up and you can sync the phone without problems. If you just want to add music without syncing other media as apps for example, check this article: