Godot 4 brings a lot of revolutionary changes. Apart from simple things like Copy/Paste for nodes in the editor between scenes and being able to have multiple windows, there's big stuff like: - the Vulkan renderer being able to scale up much more than OpenGL - Automatic LOD system that generates and automaticlaly uses decimated models to remove unnecessary geometry detail at distance - Occlusion culling, doing what BSP room culling in Godot 3 does only better and with less manual work involved (Liblast is using that to separate rooms) - also being suitable for outdoor environments - Fully real-time global illumination with SDFGI + physically-based sky texture = easy day cycle with fully dynamic indirect lighting - Volumetric fog + fog volumes that can be controlled with custom shaders - Much more powerful GPU-accelerated Particles with collision and forcefields - Fully dynamic shader-based projection decals allowing for detailing the world or adding dynamic destruction marks in the world - Overhauled networking with a new Scene Replication function, making multiplayer development even higher level in Godot 4 (we still need to update to use that one) - countless more improvements

The downsides are bugs, crashes and the fact that things still change a lot so sometimes we need to update our code to the new GDScript syntax etc :)