If you receive this error "A region must be specified when using signature version 4" while testing connection after adding the Amazon S3 credentials,
Please change the suhosin settings on your IWP server's php as below to fix the issue.
suhosin.request.disallow_nul=Off
suhosin.cookie.disallow_nul=Off
suhosin.get.disallow_nul=Off
suhosin.post.disallow_nul=Off