We’ve been periodically branching swift-4.0-branch from master, per:
The last re-branching from master to swift-4.0-branch will be June 1.
Changes will continue to be pulled into Swift 4 after that date, but the intent is for Swift 4 then to be in gradual convergence and to allow master to incorporate changes that will be included in future releases.
As far as feature work is concerned, the goal is that swift-4.0-branch contains the bulk of language and Standard Library changes core to the release by early/mid May.
Strictly additive features to the Standard Library and other parts of Swift may be considered for addition to Swift 4 even later, depending on the reward/risk they add to the release.