Enable natural scrolling in xfce4

- Run /usr/lib/gnome-settings-daemon/gnome-settings-daemon.
It can be enabled in Settings -> Session and Startup -> Autostart -> GNOME Settings Daemon

- In dconf-editor find touchpad.
It can be in org.gnome.settings-daemon.peripherals.touchpad or in org.gnome.desktop.peripherals.touchpad.
Enable natural-scroll option.

- Reboot

Note:
reverse scroll direction option in System Settings should be disabled in my case

One more solution

Uncheck the Reverse scroll direction in Mouse and Touchpad settings and put

Then, run in a terminal:

synclient | grep Delta

Search the key: VertScrollDelta and check its value. If its e.g. 58, set then it to -58 with the following command:

synclient VertScrollDelta=-58

If you want to do the same with the horizontal scroll, search instead for the key: HorizScrollDeltaand invert it in the same way:

synclient HorizScrollDelta=-58

NOTE:
This can be set at startup by adding an entry and Session and Startup > Application Autostart > Add Give some name and description and under the command type:

CODE: SELECT ALL

synclient VertScrollDelta=-58synclient HorizScrollDelta=-58

Settings would take effect on the next login.