You can't transfer a plan to a different SIM card. That is what SIM cards are for: each one contain all necessary info (subscriber's info, data/phone plan, etc.) I suggest you call your carrier and ask to get a new SIM card, the Micro-SIM which iPhone 4s use. Then you could either pop it in and start using it right away--that is, if the phone is unlocked (some iPhones are sold as 'unlocked'). However, if the phone is 'tied' to your friend's phone carrier, you may still use it, only you would have to unlock it beforehand. There is plenty of ressource which may be found on the web for that, and I suggest you look it up.
All the best,