Plone 4 is at beta 5
Progress in plone
Today Plone version 4 beta 5 has been made available for download. http://plone.org/products/plone/releases/4.0
It is not regarded officially as being of production quality, but quite a few sites are already hosted on it.
At Openia we're gearing up for Plone 4 with the imminent launch of our new virtual machine based products complete with a web accessible control panel.