I'm not a contributor to Godot, but my understanding is that Godot 3.x projects will be mostly compatible with Godot 4.0, and where conversion is needed they will provide tools.
Therefore, I assume that the fundamentals of Godot will stay the same and you should be fine diving in with 3.0 already :-)
Does it make sense to hold off diving into Godot until 4.0 (for 2D-based things) or will there not be too much to "relearn"?