I have just installed this...had it in v3.8...and have found the problem why the link isn't showing up in v4.0.3's user profile page.
In the Plugin Manager edit the plugin "Admin log in as user" with the Hook Location of "member_complete" - in there change the last line from:
Code:
$template_hook['profile_right_first'] .= $tab_html;
to
Code:
$template_hook['profile_sidebar_first'] .= $tab_html;
Seems vb in what ever wisdom they choose changed the hook names in v4.0.3