Two major differences. First, it represents the 4.0 API, so it’s a preview of what “vanilla D3” will look like sometime next year when D3 4.0 is released. Sec

Two major differences. First, it represents the 4.0 API, so it’s a preview of what “vanilla D3” will look like sometime next year when D3 4.0 is released. Second, it’s a library you can use independently of the rest of D3. So if you don’t need the other parts of D3--for example you’re using React to manipulate the DOM, or you want to render charts to Canvas rather than SVG--then you can just use d3-scale (today). Not all of the new D3 modules have been released; for example, I’m still working on layouts and selections. So for simplicity’s sake I would say that most people should keep using the default 3.x build of D3 and wait for the 4.0 release. But keep an eye on these new modules, and perhaps tinker with them, so that you’re ready when 4.0 comes out.