[kibwen]

There's some nuance to the definition of source code compatibility:

> The Swift 4 compiler will include a language compatibility flag -swift-version that accepts 3 or 4 as arguments and controls the compatibility mode (SR- 2582 <https://bugs.swift.org/browse/SR-2582>).

> The -swift-version 3 compatibility mode has the goal of source compatibility as much as practically possible with Swift 3.x (including 3, 3.0.x, 3.x).

> The -swift-version 4 compatibility allows approved source code breakages from Swift 3.x (including 3, 3.0.x, 3.x).

https://lists.swift.org/pipermail/swift-evolution/Week-of-Mo...

TL;DR: there may be breaking source changes between Swift 3 and Swift 4, but the Swift 4 compiler will still be capable of compiling Swift 3 code. There's no indication whether the Swift 5 compiler will still be able to compile Swift 3 code, so you'll likely still have to migrate your codebase within a year or two.