We're about to decide on ArcGIS API version to use. It seems that he only lacking feature for our app in version 4 is WMS/WFS support.
According to the functionality matrix, WMS and WFS are "coming soon." WMS looks like it's slotted for 4.4, the next 4.XX release, which should land in a couple months. I would treat that as an estimate, though.
Good to know. And what about WFS?
I don't see any direct mention of WFS in the JS API issues, so all I know is "coming soon."