Microsoft on Wednesday announced the rollout of new insider preview build 16251 for PC devices and 15235 for the Mobile devices. The update is currently rolling out to the fast ring insiders globally. At this point, these builds will be mostly focused on general OS improvements and bug fixes on the lead up to release.
Microsoft is actually rolling out two builds for PC today. One build is for the Fall Creators Update, and the other is for Redstone 4, the update coming after the Fall Creators Update. As of right now, these builds are essentially identical, but over time they will diverge. The real big news here is that the latest build of Windows 10 Mobile actually includes a new feature. Continuum for phones now has a new portrait mode which will be really useful for those with vertical monitor setups.