Swift 3.1 is still in convergence (having branched off already from master), but I’ve gone ahead and posted a blog post about the release management process for Swift 4.0:
The process is very similar to Swift 3.1 and previous releases. The announcement also calls out the source compatibility goals for Swift 4, with the two language modes (-swift-version 3, -swift-version 4) supported by the compiler.
The final branching date for Swift 4 is TBD, but announcements will be made as the release progress once that date gets determined. Likely at this the point the final branch date for Swift 4 would be in June, but that is purely tentative at this point.