In the What's Going On, is it possible to make Current Active Users a link
The only difference I see between 3.x and 4.0 is that the icon for Currently Active Users isn't clickable in 4.0.
However, in 4.0, the words "xxx users online" in the phrase "There are currently xxx users online. xxx members and xxx guests" is clickable. It directs you to online.php. Is this what you're looking for?