iOS 4 is the fourth major release of Apple's mobile operating system and the first under the "iOS" name — "iPhone OS" no longer fit a platform running on iPads and iPod touches, and Apple licensed the iOS mark from Cisco, which used it for router software. Released on June 21, 2010, three days before the iPhone 4, it delivered the platform's most-demanded architectural feature: multitasking.

Apple's implementation was deliberately narrow — rather than free-running background processes, apps received seven sanctioned background service types (audio, VoIP, location, task completion, push, local notifications, fast app switching) to protect battery life. Folders, the unified Mail inbox, FaceTime's system plumbing, and Retina-resolution support rounded out a release that also, notoriously, brought the iPhone 3G to its knees.

Multitasking's double-press app switcher, home-screen folders (auto-named by category), unified inbox and threaded Mail, iBooks on iPhone, custom wallpapers behind icons, Bluetooth keyboards, and a 5× digital camera zoom led the launch list. Game Center arrived with 4.1, and 4.2 unified the iPad onto the same version line while adding AirPlay streaming and AirPrint. The February 2011 4.2.x/4.3 cycle brought the Verizon CDMA iPhone's separate branch and the Personal Hotspot.

It was also the release that made updates free for iPod touch owners, ending the odd accounting-driven paid-upgrade era.

iPhone 4, iPhone 3GS, iPhone 3G, and iPod touch 2nd/3rd generations — the platform's first device cull, dropping the original iPhone and first iPod touch. On the 3G, iOS 4 ran so poorly that Apple publicly acknowledged investigating it; multitasking and wallpapers were disabled on that hardware, and the episode became the cautionary tale of stretching new OS releases across old RAM.

iOS 4 named the platform, drew its multitasking philosophy (background services over background free-for-all — a trade the industry eventually converged toward), and its FaceTime and folder features became permanent fixtures. Its 3G performance debacle produced the other legacy: Apple's since-standard practice of feature-gating heavy capabilities by hardware rather than shipping a uniformly slow OS.