IPhone 4 models. In fall 2010, pentalobe screws started to replace the Philips screws used in post-repair units in the US and in production units in Japan. The iPhone 4 shipped with iOS 4, which was the newest version of Apple's iOS mobile operating system at the time. iOS 4 introduced a multitasking system, allowing apps to remain "suspended" in memory, perform certain operations in the background, and allowing users to switch between apps using an on-screen menu. It also introduced FaceTime, which allows users to perform video calls between other iPhone, iPad, iPod Touch, and OS X users. The iPhone 4
IPhone user to have options to decline a phone call when a person is calling them. The user can reply with a message, or set a reminder to call them back at a later time. The layout of the music library is similar to that of an iPod. The iPhone can sort its media library by songs, artists, albums, videos, playlists, genres, composers, podcasts, audiobooks, and compilations. Options are presented alphabetically, except in playlists, which retain their order from iTunes. The iPhone uses a large font that allows users plenty of room to touch their selection. Users can rotate their device