Swift 4 will be ABI stable, i.e. you will be able to use Swift 4 binaries with Swift 5. So, starting in September things will really settle down, or at least, upgrades are not a problem anymore. I guess that XCode will stay compatible with Swift 4+ for a long time in the future, and it might even mean that changes in Swift 5+ will only be additions, like blocks in Obj-C were.

UPDATE: Another comment here says that code compatibility is already fixed with Swift 3+.