IFC2X4 has been in development for several years. It has now been published, actually as "Release candidate 2", which means that no more changes are expected, but the documentation is still beig refined.

Does it replace IFC2x3 ? No, it usually takes another two years for the vendors to implement each release and get their software to the market. Meanwhile buildignSMART will be using the time to test applications. For the first time, we are using automated checking, as part of our efforts to rasie the predicatability of IFC transfers.

What's new ?

The major improvements in IFC2x4 RC1 (compared with beta3) include (among others):

• enhancement and completion of the “built-in” IFC object catalogue (or classification) and can be used to map external classifications to and from. See how the IFC object definition inheritance tree represents this catalogue: http://www.iai-tech.org/ifc/IFC2x4/rc1/html/util/InheritanceTree_IfcObject.htm
• second publication of multi-lingual property sets that can be used to localize IFC property set content, currently translations exist for spatial and building elements in French, German and Japanese (new), see e.g. http://www.iai-tech.org/ifc/IFC2x4/rc1/html/psd/IfcProductExtension/Pset_SpaceCommon.xml for a quick reference. We would like encourage other chapters and interested parties to contribute to further translations!
• The process and cost element definitions have been further improved and optimized in order to reduce the model footprint, when exchanging 4D and 5D BIM. It is expected that an equivalent dataset in IFC2x4 is only 50% of the size, compared with IFC2x3. This is now accompanied with a rework of the construction resource definitions (for e.g. labour, equipment, material resources)
• More usage guidelines and illustrations are added to better explain the proper use of IFC constructs in implementations, therefore you find a number of new instantiation diagrams and summary lists (like inheritance listing, entity/property set listings, etc).

More information ?

The IFC2x4 RC1 release can be characterized as:

• 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.
• Get a quick overview from "unveiling IFC2x4" at http://www.iai-tech.org/downloads/accompanying-documents/presentations/Unveiling%20IFC2x4.pdf
• See the "Whats' new",  http://www.iai-tech.org/ifc/IFC2x4/rc1/html/change/IFC2x4-rc1_whats_new.htm, section for a quick overview
• See the "IFC2x4 RC1 change log" http://www.iai-tech.org/ifc/IFC2x4/rc1/html/change/IFC2x4-rc1_change-log.htm for a detailed register of all changes. 
• Go to http://www.iai-tech.org/products/ifc_specification/ifc-releases/ifc2x4-release/rc1-release for the IFC2x4 RC1 announcement and follow the links for online access or download.
• For more information visit the official technical website of buildingSMART www.iai-tech.org (also linked via) http://ifc.buildingsmart.com