The iOS 4 is now available for download via iTunes and the file weighs in at 378MB. To download iOS4, you have to first upgrade your iTunes to version 9.2.

For Unlocked And Jailbroken iPhone

For those using unlocked or jailbroken iPhone, please DO NOT upgrade to iOS4 for now. You need to stay from it for a while until iPhone Dev team releases both the jailbreak and unlock solution. Though there is no mention about the release date, I expect the tools will be available in these two weeks.