Version 4.0 of the GTK toolkit has been released. “It is
impossible to summarize 4 years of development in a single post. We’ve
written detailed articles about many of the new things in this release over
the past year: Data
transfers, Event
controllers, Layout
managers, Render
nodes, Media
playback, Scalable
lists, Shaders, Accessibility.” GTK 2 has reached the end of its life.

Source: LWN.net – GTK 4.0