Godot 3 has more "goodies" available (for now) around the web and in the Asset Library, but that's changing quickly. Godot 4 has some more modern features and other improvements which (depending on your project intent) may make it the better choice. It honestly depends entirely on what you're attempting to create, but at this particular point in time either one will likely get the job done. Sooner or later Godot 4 though will be the preferred version to choose, so it can't really hurt to start learning it now. :)