Xcode 9.4 has been deprecated for a long time and you won’t be able to submit new app versions to App Store Connect using it (nor Xcode 10 for that matter!).
The only possible reason I can think of for trying to use Xcode 9.4 today is to use the Swift migrator to upgrade some really old Swift code. You’re either going to have to upgrade the code manually, or install an older version of macOS in a virtual machine so that you can then install Xcode 9.4 inside that. Good luck!