The F4 key is usually in the top row of keys:
picture
source: http://www.webdevelopersnotes.com/articles/laptop_touchpad_not_working.php
Also see:
http://www.computerhope.com/issues/ch000306.htm
and http://windows.microsoft.com/en-us/windows/using-keyboard#using-keyboard=windows-7
Hope that helps :)
Cheers,
J