You can compile and run 4.9 apps  and use them and even mix and match some of them with 4.8.

When you build the 4.9 panel use a prefix of /opt/xfce4.9.  "configure --prefix=/opt/xfce4.9" then build and install.

In your .xinitrc file exec startxfce4 like normal or leave alone and log in with a display manager (gdm, lxdm etc).

Start Xorg. Remove the xfce-panel.

Now, goto Settings -> Session and Startup.

Click add, add /opt/xfce4.9/bin/xfce-panel.

Log out / back in to X.
-JimB.

I'm using Archlinux because it is rolling release but really, I'm using Debian. That is why I want to know, I need to get the latest version of Xfce, without waiting for the Debian repositories.

I guess I need to compile Xfce. But I have a question: Can I use xfce4-panel 4.9 with Xfce 4.8?