The big change in this build is High Efficiency File System. Here's what Microsoft says about it:
The High Efficiency Image File Format (HEIF) has arrived for Windows 10 and the Photos app! HEIF is an image container that leverages modern codecs like HEVC to improve quality, compression, and capabilities compared to earlier formats like JPEG, GIF, and PNG. In addition to traditional single images, HEIF supports encoding image sequences, image collections, auxiliary images like alpha or depth maps, live images & video, audio, and HDR for greater contrast. We have heard your feedback that these features and the ability to share photos easily with other platforms is important to you. This release we are delivering essential viewing support (You cannot edit HEIF images in this release).
In order to try this out, you need to join the Windows App Preview Program for the Photos app and make sure you are running the March release of the Photos app (Version 2018.18022.13740.0 or newer). This version of the Photos app has been updated for RS4 to support viewing the primary image inside a HEIF file and to guide the install of dependencies like the HEIF and HEVC media extensions from the Microsoft Store. Once installed, these media extensions enable HEIF viewing in Photos as well as thumbnails and metadata in File Explorer.
Here's the full release notes.