Would a T-mobile sim card work in a Sprint iPhone 4s?

I was going to buy my friends sprint iphone 4s and was wondering if I'd have to buy a new sim card? If I could use my t-mobile one though, that'd be great.

I've had an iphone 4s and you can't put a sim card into one.

I don't think you can use them interchangeably. Tmobile uses GSM frequencies, and Sprint uses CDMA so if you buy a Sprint phone, you have to use it on Sprint. If you buy a Tmobile phone, you CAN have it unlocked to be used on other GSM networks like AT&T or Family Mobile, etc.