@amisner2k summarizes most of what I know about the Godot 4.0. Last I knew the idea was that Godot 4.0 was going to have a fair amount of changes within the engine, but not too much different for end users.
The transition to Godot 3.0 to Godot 4.0 is supposed to be smoother and less jarring than the transition from Godot 2.0 to Godot 3.0. There is a proposal to rename some of the nodes for better consistency, but the old names will be deprecated for awhile so everyone has time to transition.
If you look at GitHub, under the 4.0 milestone, you can get a rough idea of what changes might be coming with Godot 4.0. Juan/Reduz posts development posts on his Twitter occasionally, which is one way I keep in touch with the big changes happening with Godot.
Right now though, most of the focus for Godot 4.0 is on the Vulkan integration. The changes to the core engine should bring some nice performance improvements, especially on the 3D side. I remember reading that the 2D side of the Vulkan implementation is mostly finished, and from what I have seen the 3D side is making good progress.
Outside of the Vulkan implementation, it is hard to say at this point what will be included in Godot 4.0 or not. I could speculate, but honestly I have not kept up enough with the changes to make more than guesses.
@uriel said:
... I am very curious to discuss the plan, the dates and the specs.
Most of Godot's development is being spearheaded by Reduz, who is not very active on these forums. He probably knows the most about where Godot is going and other development related specifics. As far as a release date, I do not think there is any date set for the release of Godot 4.0 just yet.
If you are curious on the more low-level stuff, I think most of the engine development related talk happens on the engine development IRC. You can find a list of all of the Godot community sites, including the IRCs channels, on the Godot documentation.
