How can I switch to Swift 4.0 in Xcode 9.3?

It seems like I have only options to go to 3.3, or unspecified:

enter image description here

Can I switch to 4.0 somehow?

How do I upgrade my version of Swift?

You can also update every single Swift Package in your project at once. To do this, open the Files menu, navigate to Packages and then click Update to Latest Package Versions. Afterwards, make sure to test your project to see if everything still works, especially in projects with a lot of package dependencies.

What version of Swift does Xcode use?

This book describes Swift 5.7, the default version of Swift that's included in Xcode 14.

Make sure you have updated xCode then under Build Settings, check the drop down options for Swift Compiler - Language.

Furthermore, You can download Xcode 8.x and 9.x from Apple Download Portal, if you've premium developer account (apple id). You can install & work with both Xcode 9 and Xcode 8.x in single (mac) system. (Make sure you've Command Line Tools supporting both version of Xcode, to work with terminal)