Hi GIS Mountains,
> Because the 4.x 2D API is still so lacking (editing tools, drawing tools, and much more) WAB will not be switching (based on info I have) until the 3.x and 4.x API are much closer in parity.
Robert Scheitlin, GISP's comment is correct. We need to wait until for functionality equivalency in the ArcGIS API for JavaScript v4.x to match v3.x, before we can migrate Web AppBuilder for ArcGIS to just use the v4.x API.
> There is no official or yet un-official time line that has been mentioned.
Unofficially, the JavaScript API Dev team is targeting the end of 2017/Q1 2018 for the ArcGIS API for JavaScript v4.x to match v3.x. It's likely that we would need some time afterwards to migrate Web AppBuilder to just use the v4.x API.
Hope this helps,