See the release announcement (2019-09-16) and the release tag on GitHub. To be done once we're on v3 (#3649).

There's a handy v3 -> v4 upgrade guide for this, here. (I haven't found one for the v2 -> v3 upgrade.)

It appears that FlowTyped does have a libdef for v4! So it's very likely that we won't have to use Flowgen for this upgrade.