Firstly I do want to say thank you for releasing this as it's been a long time waiting for some bug fixes.

I've just started testing the new forums before updating the live site and some things I've noticed are:

• HTML decoding with subject in notifications (eg '&' in a forum name/subject line is still shown as '&')
• Forums show most posts with ugly text "Go to first Unread Post" just before the last posts date and user for the thread.
• Icons at the top still jump all over the place with the display, from vertical to horizontal and all mixes inbetween
• Admin nav is better to use, unclear at first what is a link with no hover cursor - but this nav also jumps around with the formatting and makes it difficult and unclear what is selected (occassionally has large text appearing to left "Selected Menu Item", what options are there - just all over the place.  Noticed this most in "My Settings".

It does seem nicer to use, but the mess it's creating does leave me unsure whether upgrading the live site is a good idea or not.  I really just hate the thought of waiting nearly 2 more years for these fixes.

I would upgrade to 4.9.4 or even 5.1.1 when it is out, and get it checked out on the latest platform.

I'd be intested to hear if anybody else gets these notification issues..... are you running a webfarm?

Check out the long list of resolved issues in the change log:
http://support.dotnetnuke.com/project/ChangeLog.aspx?PROJID=7

The testing goes against NN 4.9.4 without a webfarm.  I had never seen this issue.  So some more informaion about your environment and perhaps after upgrade to actually version could be helpfull.