Yes, that’s the plan. “Soon” may be an overstatement, but we do plan to release GTK4 later this year.
Clutter was developed in 2006 with the goal to be a technological demonstrator for writing full screen media players on embedded and resource constrained devices, like the old ultra-mobile PC platforms. After 2007-2008, it got repurposed as a way to create new, mobile user interfaces, using hardware acceleration (GL and GLES) and providing an animation framework to create new UI toolkits. Additionally, it was used to write desktop shells, like Moblin and GNOME 3.
You can think of Clutter as a prototype for many of the design choices that are now being used by GTK4:
- explicit scene graph API, like the Web’s DOM, instead of implicit API like GTK1, 2, and 3
- deferred rendering through a retained tree of paint primitives
- layout management decoupled from scene graph elements
- delegation to ancillary objects instead of deep type hierarchies
In general, Clutter is meant to be a mid-level toolkit—a library to write toolkits based on its primitives. That design choice was not great, though, as it came with overgeneralisation and performance trade-offs. It was a good experiment, though, because that allowed GTK4 to concentrate on being a proper toolkit, not something that can be manipulated out of tree by poking at its internals.