swift 4 or 5 Question

You don't mention any of your goals, details, etc. on why you want to back up to Swift 4.x

Otherwise, you've answered your own question.

I do not see immediately the intesrest of doing so, but you can change Build settings (In project and/or Taget sction) to Swift 4 or Swift 4.2.

Depends if you've used some specific Swift5 features.