When you add a repository, what that does is changes where 'apt install' gets its stuff from.
ex.
apt install xfce4-zorinmenulite-plugin
You got:
E: Unable to locate package xfce4-zorinmenulite-plugin
Then you run this command (this might not be the actual command):
sudo add-apt-repository ppa:xubuntu-dev/staging
Now if you run:
apt install xfce4-zorinmenulite-plugin
It works because its getting the plugin from a different location. In summary, your still on 4.14 because you 'apt install xfce4' BEFORE you added the repository. Now that you have the repository, you would have to reinstall xfce4. Actually.. now that you have the repository... you might just have to do an update and thats it.
apt-get update -y && apt-get upgrade -y





