A lot of the xfce related packages are named xfce4-* eg. xfce-base/xfce4. Gentoo policy is to name packages without embedding version numbers.
That's how upstream named it, beside that we even have whole categories with major versions in their name (dev-php[45]), won't change that.
php was split because the php4/php5 packages were incompatible and the maintainer said there was no was to mix them. This is a different thing.
No, we won't be changing the ebuild names; the desktop environment is called Xfce4 and version is 4.4.1.