I've been testing 4.0 for a while now, and this is what I've stumbled upon:
- When I enter the anti-spam-code I have to click the send-button. If I just hit [Enter] the entire comment is lost, vanished into thin air.
- Sometimes (and sorry, I don't know when and when not) when I hit the send-button, nothing happens. When I enter another comment the first and second one appear.
- Most of the time it doesn't accept the entered captcha-code (and yes, I can type it correctly)
- The notify by email doesn't work
- It still says 'Powered by !JoomlaComment 3.25' in the footer

I've tested all the template-combinations on three different test-sites of mine (both hosted local and remote), they all work.
Haven't yet tested the RSS - sorry.
I hope this helps you in any way. You can always throw questions or remarks my way!
Guido Hulscher
EDIT:
I forgot the most important: managing comments gives this error:
Catchable fatal error: Object of class stdClass could not be converted to string in C:\xampp\htdocs\joomla\administrator\components\com_comment\admin.comment.html.php on line 50