Today Microsoft pushed the fourth Redstone 4 release build out to the fast ring. This is only 2 builds newer than this week's previous build, so what exactly is new in this release?
Um, actually .... nothing!
This build is probably best avoided to be honest; it introduces a fairly big Mixed Reality bug which causes Mixed Reality to run very slowly and another which means the computer might blue screen (green screen as it's actually a test build) when a document is not sync'd from OneDrive; and it doesn't fix the disappearing Microsoft Store bug from 17110.
All in all, I can't recommend this build. The full release notes are here.