I think as long as the SIM card supports 3G, you should be fine.
It works for my SGS on M1 Prepaid. You can either buy a micro SIM
cutter or cut the SIM card yourself. You can't go to Singtel and
ask them for a micro SIM for your prepaid SIM.
Don't listen to him!Once your SIM card is cut, it cannot be used
anymore.I think it will work but if you can afford iphone 4, why
still use prepaid card?