works on my system without even restarting the wm, just so you know it should work.
what's the value for double clicking the window in ~/.config/xfce4/mcs_settings/xfwm4.xml (if FC4 follows the new ~/.config standard, otherwise, it will be somewhere else depending on distro)? it should say maximize, e.g.:
<option name="Xfwm/DblClickAction" type="string" value="maximize"/>