Oh, my bad. It should've been 'client_credentials' in the screenshot, not 'client_authentication'. Anyway, it didn't work.
Thanks for the clarification, David. In this case, it looks like you don't need to use OAuth at all. Just make the same request as you did with version 4 in order to obtain the token. Extract the token from the response (e.g. using jsonvalue()) and use it in the consequent requests in the workflow with the "Authorization" header as follows:
Authorization: Bearer <your token>
Notice that you should add Bearer in front of the token (notice the space between "Bearer" and the token).
The calculated header value can be used in the "Web request" action using a first column value, or a parameter.
According to the screenshots, the token is valid for 1 hour. Therefore if your workflow takes less than 1 hour to accomplish, this approach should work because every workflow run will obtain a new token.