Jailbreaking an iPhone will void its warranty with Apple, instead consider an official unlocked iPhone 4 from Apple itself. Price – 16GB unlocked iPhone 4 is $649 for and 32GB is $749. You can use the unlocked iPhone 4 with any GSM phone carrier which supports micro-SIM cards. You should contact your GSM carrier for a micro-SIM card.
Will a unlocked/jailbroken iPhone work on Verizon Wireless or Sprint Wireless in US?
Answer is No.
You cannot use your iPhone in Verizon or Sprint network as there are not GSM carriers. Sprint and Verizon wireless uses CDMA technology they don’t use SIM cards. AT&T Wireless and T-Mobile USA are the chief GSM carriers in the United States. GSM and CDMA phones are not compatible with each other.
Advantage of buying an unlocked phone is that you can use your new phone with the service provider of your choice. A smartphone will allow you to do more than make phone calls, offering advanced capabilities, often with PC-like functionality.