The first Redstone 4 test build for HoloLens is Build 17123, which is a few builds older than the latest version of Redstone 4 that Insiders are testing on PCs.
As noted in the HoloLens RS4 Preview documentation, dated March 23 on the Microsoft documentation site, this test build is available to all HoloLens users. However, installing the preview will result in the erasing of all users' content and restore the device to factory settings.
To install the preview, users first need to grab the latest version of the Windows Device Recovery Tool, and then they need to make sure their HoloLens goggles are enrolled in the Windows Insider Program.
File Explorer support for moving and deleting files from within HoloLens
Media Transfer Protocol support for allowing desktop PCs to access photos, videos and docs on HoloLens
Ability to set up HoloLens on a guest network using captive portal
The test build also includes some features specific to developers around spatial mapping and app tailoring interfaces. It also has some features specific to commercial customers around multiple Azure Active Directory user support, mail sync and more, and some for IT pros around setup, diagnostics, and more.