iPhone OS 4.0 has been renamed to iOS 4.0. What does it mean? It is because iPad has hit shelves and it uses the same OS althouogh it should have been renamed very earlier because iPod and iPod Touch uses same firmware. iOS 4.0 brings a lot of features for iPhone users. iOS 4.0 enables you to run multiple apps at the same time which is actually called Multitasking in technical terms. It means if your iPhone is equipped with iOS 4.0, you can browse web while listening to music or check an email without shutting down camera app.
iOS 4.0 supports organizing installed apps in folders which ultimately allows to have more and more apps on iPhone or iPod Touch. There are other various features introduced in iOS 4.0. Does this eliminate the need to jailbreak iOS 4.0? A big NO! The internet tethering is not possible without jailbreaking iOS 4. Jailbreaking is also a prerequisite to unlock any iPhone. So, unlocking iOS 4.0 will also require to jailbreak iOS 4.0. There is also a plethora of apps developed for jailbroken device and many users are addicted to them. Unfortunately, Apple’s control over apps approval will still push users to jailbreak iOS 4.0.
As iPhone 4.0 firmware has been only renamed to iOS 4.0, the jailbreak 4.0 saga which has been happening since beta release of iPhone 4.0 firmware holds true.
You can read the following articles which mentions the beta, upcoming and announce tools to jailbreak iOS 4.0
Update 1: limera1n jailbreak may be the next tool by GeoHot. It is expected that it will be jailbreak 4.0 firmware for iPhone,iPod Touch and iPad 3.2 but nothing can be ensured till limera1n download is available or any official information is released.
Update 3: Kaatje has managed to jailbreak 4.0 on iPhone 3GS. It may be released by iPhone Dev Team as Pwnage Tool 4.0 but nothing can be assured until Pwnage Tool 4.0 download is available
Update 4: You can watch a working demonstration of exploit which can jailbreak iPod Touch 4.0
Update 5:Unlock iPhone 4.0 is discussed with some expected tools to achieve the task.