In the eyes of many friends, the iPhone 4 may be the most classic Apple mobile phone, and its iOS 4 is also full of memories. Recently, overseas developer Zane made this 11-year-old system into an APP called OldOS. The developer used SwiftUI to restore the design of iOS 4 that year and realized full functionality. According to the developer, you can even use it as a second system. However, OldOS also has some problems. For example, there are major bugs in the YouTube APP, and there is no way to use it temporarily. Currently, OldOS is passing the TestFlight test. However, as of the time of publication, the number of applicants is full, and it is temporarily impossible to confirm whether a new round of testing will be conducted. It is reported that this is not the first such "memory kill" application in the Apple ecosystem. The previous APP called Rewound can simulate iPod Classic on iPhone and provide music playback function. However, Apple doesn't seem to support this approach, and Rewound was taken off the shelves within a week of its launch. Therefore, it is currently uncertain whether OldOS can successfully log in to the APP Store.