HI if the Samsung captivate phone is locked to some other network provider then only by unlocking your phone you can freely use it with old sim card which you used in iphone 4 ..You can easily get it unlocked using sites like Mobile-Unlocker.com they are providing unlock code for all mobile models at reliable cost .....You first verify that whether the code is available for your country and network provider .If yes then pay for it and unlock your phone ..After unlocking the phone you can freely use it with any GSM sim .....