There should be a possibility to define a screen locker user specifically and it should not depend on a dedicated system installation that only the admin can enforce.

This should be easily fixed via a few lines added into xflock4 script to remember the preferred locker in a user's xfconf entry. That would not interfere with the classic logic of prioritizing known lockers, e.g. xscreensaver as the very best.

See also bug #9935, bug #10217, bug #10679, bug #11254, bug #9063, bug #8993.

$ sh -x xflock4
+ PATH=/bin:/usr/bin
+ export PATH
+ for lock_cmd in '"xscreensaver-command -lock"' '"gnome-screensaver-command --lock"'
+ xscreensaver-command -lock
+ for lock_cmd in '"xscreensaver-command -lock"' '"gnome-screensaver-command --lock"'
+ gnome-screensaver-command --lock
+ for lock_cmd in '"xlock -mode random"' '"slock"'
+ set -- xlock -mode random
+ command -v -- xlock
+ xset dpms force off
+ xlock -mode random
+ exit