Open /usr/bin/xflock4 as root and comment out this line:
LOCK_CMD=$(xfconf-query -c xfce4-session -p /general/LockCommand) #put a # in beginning of the line
Install xfce4-screensaver:
sudo pacman -S xfce4-screensaver
Open the Screensaver app from the launcher. It will prompt you the first time to enable the service so click Yes and done 