@TheDiavolo said:
I won't be able to test it, since my PC is not compatible with Vulkan and I won't be able to buy a better PC for a long time
The goal is to feature an OpenGL renderer alongside the Vulkan renderer, so most hardware will still be able to run Godot 4.0.
Many backwards-compatible improvements were already backported to 3.x, but backwards-incompatible features such as the new TileMap editor won't. Otherwise, compatibility with existing projects would be broken.