1. You can use Swift 4 with any version of iOS 7 and above . Note: as iOS 11 is dropping support for 32-bit, you will only be able to use iOS 11 from an arm64 slice (that is, from iPhone 5S and up). If an iOS target has an iOS Deployment Target of 11, then it will not build for 32-bit. iOS targets with an iOS Deployment Target earlier than 11 will still build for 32-bit by default. 2. Swift 4.1, the default version of Swift that’s included in Xcode 9.2 . You can use Xcode 9.2 to build targets that are written in either Swift 4 or Swift 3. Swift 4 can depend on a target that’s written in Swift 3, and vice versa. This means, if you have a large project that’s divided into multiple frameworks, you can migrate your code from Swift 3 to Swift 4 one framework at a time. 3. Compatibility with Xcode 9.2 Xcode 9 requires a Mac running macOS 10.13.2 or later . Xcode 9 includes SDKs for iOS 11.3, watchOS 4.3, macOS 10.13.4, and tvOS 11.3.
Think big, start small, move fast.