In previous versions of xflock4 you could specify some paramaters to pass to the lock program, this seems to be no longer the case with 4.10.0 Downstream report at: https://bugzilla.redhat.com/show_bug.cgi?id=923393 It would be nice if users could pass arguments on again.
If I remember correctly, arguments applied to xlock only, so why don't you call xlock directly? xflock4 is desinged to work with various screenlockers, and it is supposed to lock access immediately.
See also https://bugzilla.xfce.org/show_bug.cgi?id=6516
See also bug #11324.
I agree with Jarno. Additionally, the default locker can now be easily set via the /general/LockCommand property in the xfce4-session channel of xfconf. This custom command will be then executed by xflock4.