Although probably not recommended just yet (early doors and all that) the Xubuntu Development PPA contains pre-release builds of Xfce 4.12 for Ubuntu 14.04 LTS and 14.10 users to install from at their own risk.
Add the Xubuntu Dev PPA to Ubuntu’s Software Sources using a Terminal and the following commands (assuming the Xfce desktop is already installed):
sudo add-apt-repository ppa:xubuntu-dev/xfce-4.12
sudo apt-get update && sudo apt-get dist-upgrade
To remove the PPA and downgrade to the version of PPA in the main Ubuntu repos/other installed PPAs the ‘ppa-purge’ utility should be installed and used:
sudo apt-get install ppa-purge
sudo ppa-purge ppa:xubuntu-dev/xfce-4.12