Hi,
Another newbie here. The 404 error happened to me too, immediately after upgrading to WP 4.0.
I took your advice about refreshing the permalink settings and it almost didn’t work. That is, it didn’t work since I hadn’t made any changes.
But then I made a simple (for me) change: I changed the Common Settings from ‘Default’ to ‘Day and name’. That seems to have done the trick, the forum is now working. Note that when I changed it back to ‘Default’ the problem occurred again, so I’ll stick with “Day and name” for now.
