Swift 4? Wow, they surely are iterating quick... How's the backwards compatibility in Swift between major versions? If I understood correctly, Swift 3 was supposed to be the first production-ready release. Or am I wrong?
Swift 3's goal was source stability, but Swift 2 and arguably Swift 1 were production-ready. Swift 4's goal is ABI stability, as well as source compatibility (via a compiler flag) with Swift 3 code. In any case, Swift is basically incrementing by one major version per year. Swift 1.0 was September 2014, Swift 2.0 was September 2015, and Swift 3.0 was September 2016.
People shipped with Swift 2. There were lots of source changes in 3. XCode converted many. 4 will have few changes but it should be an easy conversion. Personally, I love Swift. Creating my own Swift Cookbook to help myself and others find solutions quickly.
Swift 4 has source-level compatibility with Swift 3 though a compiler flag. Some features obviously won't be available but existing code should work. This even goes so far as to present APIs differently so they continue to work (e.g. if a method was renamed Swift 3 mode will continue to offer the old name).
Swift 4 also has ABI stability as a goal so the OSes can begin shipping the Swift Standard Library.