With yesterday's release of Xcode 9, all 11 books in the Hacking with Swift series have been updated for Swift 4. These updates are available free of charge thanks to the Hacking with Swift lifetime update policy, which guarantees you Swift updates for the life of each book.
If you bought through Gumroad: The book updates are available immediately using your original email receipt. Look for the product download link from Gumroad, and that should take you straight to the updated editions.
If you bought through iBooks: The updates have been submitted to Apple for review and will hopefully be live in the next few days.
If you're keen to learn more, here are links to each of the books:
At the same time as updating the books for Swift 4 some other enhancements were added, including:
Never, and Swift 4 dictionary improvements.
As well as the books being refreshed, the Swift Test has been updated for Swift 4, and the Swift Knowledge Base has been updated for Swift 4 and expanded with new articles relating to iOS 11 and iPhone X. The new articles are:
Paul Hudson is the creator of Hacking with Swift, the most comprehensive series of Swift books in the world. He's also the editor of Swift Developer News, the maintainer of the Swift Knowledge Base, and Mario Kart world champion. OK, so that last part isn't true. If you're curious you can learn more here.