Anyone testing xfce4-4.10 please note that you will be able to downgrade to 4.8.0, but it has the potential to create a lot of conflict in your system.
You may want to do this because xfce4-4.10 does not yet have a full set of working plug-ins, but better to hold on and wait for the new plugins to arrive.
xfce4-4.8.0 is a single txz file containing every component. xfce4-4.10 is a meta file which pulls the separate component txz files as dependencies.
When you upgrade to xfce4.4.10- it works fine.
when you downgrade, xfce4-4.8.0 installs but only the meta file for xfce4-4.10 is uninstalled and all the xfce4-4.10 files remain mingled with your 4.8.0 files. This causes some function failurs but surprisingly is still a usable system.
If you try to upgrade again to xfce4.10, then xfce4-4.8.0 then your system will brick your machine, for xfce.
The reason is not clear to me as yet, but if anyone cares to throw it in a virtual machine and give it a look I'd appreciate it.