Quick details (courtesy of IAI Tech International):
The IFC 2x edition 4 release is available now. The buildingSMART community is strongly encouraged to review the current alpha release and to provide feedback using the MSG issue resolution database. The deadline for comments is 20.08.2008.
The IFC 2x edition 4 release (IFC2x4), combines a number of feature increases with some major rework and improvements of the existing IFC specification. It has been developed as the next basis for IFC enabled interoperability of Building Information Models. It is also intended that the IFC2x4 release will be submitted to the International Standardization Organization (ISO) for approval as a full International Standard ISO16739.
The spec is available for download here: http://ifc.buildingsmart.com
Note: You will need to register for free to be able to download the file and provide feedback. It looks like new registrations are currently down and should be back up soon.