GTK 4.0 Toolkit Officially Released [phoronix.com]
GTK 4.0 features new widgets and reworks to existing elements, integrated media playback support [phoronix.com], GPU acceleration improvements like work on its new Vulkan renderer [phoronix.com], and better macOS support are some of the leading highlights. Some other additions include data transfer improvements [phoronix.com], overhauling shaders [phoronix.com], GPU accelerated scrolling [phoronix.com], custom entry widgets are easy to make [phoronix.com], OpenGL rendering improvements [phoronix.com] beyond the Vulkan work, restoring work on HTMl5 Broadway [phoronix.com], better Windows support [phoronix.com], and more.
GTK 4.0 is now considered stable for applications to begin supporting it. GTK 3 will continue to be maintained for the "foreseeable future" while GTK 2 is no longer going to be supported beyond one more point release.
GTK 4.0 [gtk.org].