Apart from the obvious (being rs4_release rather than rs4_development), this build introduces quite a few new features.
The first of these are... emojis. Yes, really. Apparently these are a major OS feature now! That's it; I don't intend to mention more about them.
Far more interesting are the changes which have happened to Windows Universal App file permissions. It is now far easier (and more granular) to decide what file system permissions a UWP app can have, thus determining what it can access. As Microsoft say:
With this build we’re giving you more control so you can now decide which UWP apps can access your full file system. Particular UWP apps will be granted permission to have broad file system access. This capability will be granted on a per app basis by Microsoft. If a UWP app has the broad file system access restricted capability, you will receive a consent dialog prompting you to accept or deny the request. If at any time you change your mind about the decision, you can go to Settings > Privacy where you’ll find a new Settings page for File System Access. On this page, you can turn access on or off globally, and if it’s on you can also turn it on or off for each app that has requested the capability. UWPs with broad file system access will not appear in the Photos, Videos, or Documents privacy settings pages. If you grant broad file system access, this includes Photos, Videos and Documents.
In addition to this, there's some "productivity" changes to Windows 10 Pro for Workstations, which basically means MS have stopped bundling all the Candy Crush crap on that particular SKU and instead installed some useful apps; and finally, there's a new power plan setting for Workstations called "Ultimate Performance" which, um... gives you ultimate performance on that hardware.
As ever, the full release notes are available here.