The current major release of IFC, superseding the long-dominant IFC2x3 version. IFC4 added support for richer parametric relationships, better structural analysis data, infrastructure elements, and tighter alignment with openBIM workflows. In practice, IFC2x3 is still widely requested on many projects simply out of software-compatibility habit, so it is common to see an EIR specify exactly which IFC version - 2x3 or 4 - a project must use, since the two are not fully interchangeable.

Knowing to ask "which IFC version?" rather than assuming "IFC" means one single thing is a small detail that saves real coordination headaches.