Created attachment 5303 I used the current file from git as original in this patch. Xscreensaver should be used, if it is running.
The patch also removes some extra whitespace, so that is why it is so big.
xflock4 shouldn't fail if xscreensaver is running, so I'm not sure I understand this bugreport. Both gnome-screensaver and xscreensaver are supported by xflock4. Also since the choice of locker is configurable now via an xfconf property in xfce4-session, I would say this bug can be considered fixed. Please reopen if deemed necessary and argue why this is a bug in xfce4-power-manager.
For example Lubuntu linux has xfce4-power-manager, but not xfce4-session, so it can not use xflock4, except if such a script is manually added. LXDE uses lxlock (via lxsession package), instead, for locking.
That's true, however this is only a transitional problem since Lubuntu will eventually switch to LXQt.
I think it is not wise to have gnome-screensaver as the first option. It is less secure than xscreensaver, at least in GNOME. I am not sure, if this applies to other desktop environments as well. http://www.jwz.org/blog/2015/04/i-told-you-so-again/
Currently xfce4-power-manager tries to query several screensavers via dbus and only then falls back to calling them on the commandline. xscreensaver is in the list, so this should work for Lubuntu (even without xflock4). Also it does not really matter in my opinion which locker is mentioned first. If you use a distribution you get a working default setup (and xfpm will work in pretty much whatever setup). If you decide to install both gnome-screensaver *and* xscreensaver and run them at the same time (which is really the only time when this "priority" makes any difference) you should really know what you're doing anyway.