I am afraid there is no way at the moment to easily provide a single feedback that would be automatically available to all the 4 students. You will have to copy & paste it manually for now.
It seems like a good and valid feature request though. I can imagine a checkbox like "Make the feedback available for peer-reviewers, too" or something like that. Should that be controlled on the whole Workshop level? (so that it applies to all authors and reviewers equally) Or would it be better to have it configurable per submission? (so that the teacher would have the checkbox next to each individual feedback.