What new features are offered by iOS 4? Which iPhone, iPod touch, and iPad models are supported? How much does it cost? Is it free?

Please note that the current version of the iOS is iOS 5. This Q&A remains for "historical" reference.

As dutifully "live blogged" by Engadget, Gizmodo, the WSJ, the New York Times and many other sites and summarized on Apple's own site as well, there are over "100 new user features" in iOS 4 (originally named "iPhone OS 4" when introduced on April 8, 2010 but renamed "iOS 4" on June 7, 2010 when the iPhone 4 was unveiled).

Apple considers seven new features to be the most important. These include:

Multitasking -- Allows users to switch back-and-forth between multiple applications by double clicking the "home" button. All currently running applications are displayed and each can be accessed with a single touch. As first spotted by Gizmodo, this user video submitted to YouTube demonstrates the process:

Folders -- iOS 4 makes it possible to have up to 2160 applications in effectively the same manner as a traditional desktop computer hierarchy. However, each folder is displayed as an icon with a grid of smaller application icons "within" it rather than as a filing cabinet-inspired folder icon. Folders can be created on the fly on an iPhone, iPod touch, or iPad and managed from within iTunes on a Mac or Windows PC as well.

Improved Mail -- As Apple explains, the Mail software in iOS 4 makes it possible to "see messages from all your email accounts displayed together in a unified inbox, switch between inboxes more quickly, organize messages by threads, and even open attachments in third-party apps."

iBooks -- Just like the iPad, iOS 4 provides an iBooks application to read e-books on the smaller iPhone and iPod touch models.

Enterprise Enhancements -- Promises "enhancements to security, scalability, and compatibility" for businesses.

Game Center -- A "preview" of a social gaming network that will allow users to play against one another as well as "invite friends," match players for multiplayer games, and keep track of "leaderboards" and "achievements" for gamers.

iAd -- A "breakthrough mobile advertising platform" that will essentially allow advertisers to embed "mini-applications" within iPhone apps that choose to support the iAd advertising system. Apple provides 60% of the advertising revenue generated to application developers.

On June 7, 2010, Apple quietly added Bluetooth keyboard support to the list of iOS 4 features, another welcomed addition for those interested in using a keyboard with an iPhone or iPod touch.

iOS 4 supports the iPhone 4, iPhone 3GS, 32 GB and 64 GB configurations of the third generation iPod touch, and the iPad models completely. iOS 4 supports the iPhone 3G, 8 GB configuration of the third generation iPod touch, and second generation iPod touch as well although Apple notes that "not all features are compatible" including multitasking. The original iPhone and original iPod touch are not supported at all.

When this Q&A initially was published on April 8, 2010, EveryiPhone.com speculated:

Apple has not revealed pricing for iPhone OS 4, although it is expected to be free for iPad purchasers and historically Apple has released operating system updates for free for iPhone users and charged iPod touch users.

However, the March 17, 2009 press release for iPhone OS 3 specified "pricing and availability" and directly noted that the operating system upgrade would be free for iPhone users but cost US$9.95 for iPod touch users. The April 8, 2010 press release for iPhone OS 4, by contrast, merely states "availability" and declines to mention whether or not it will be free for iPhone users.

Consequently, as "free" is perhaps the most powerful word in advertising lexicon it is unlikely that it was omitted without reason and the safe assumption is that iPhone OS 4 will be a paid upgrade for all compatible iPod touch and iPhone models. Should this assumption be incorrect, or Apple release additional information, this Q&A will be updated.

As it turned out, however, this speculation pleasantly was incorrect. On June 7, 2010, upon introducing the iPhone 4, Apple also announced that iOS 4 was a free upgrade for all compatible iPhone and iPod touch models. Particularly for iPod touch users, this break with the historical upgrade pattern is a delightful surprise indeed.

Also see: Which iPhone, iPod touch and iPad models fully support iOS 4? Which models partially support iOS 4? For models that "partially" support iOS 4, which features are and are not supported?