You can Unlock the I-phone 4,after that You can use it any GSM Network as you wish...Ok...But please specify the Current Network,Country the Mobile Locked?
Because all Network locked Phones cannot be Unlocked ,only list of Country provide Unlock services,You can Unlock it remotely using the Remote Unlock service,You can get the remote I-phone Unlock service from here
Superunlockcodes and unlock Your I-phones simply as the site name,the method also simple.....all the best.....then Jailbreaking is Must...Because without Jailbreak You cannot install the other third party application in your Phone...You need to jailbreak to use Full features of those i-phones...You can jailbreak your i-phone here
idownloadblog.com/jailbreak