When I give this a try with the 4.0.1 release I'm able to edit the message without issue. I did notice that the way the template was applied to this page seems different than in 4.0.0 but it did not prevent me from making any changes to the content page.
I have opened a ticket for you on this issue so that it can be looked into further. To update your ticket or check on it's status login and visit your support history.
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