Late (east coast time) on January 11, Microsoft rolled out an updated Windows 10 Redstone 4 test build to Fast Ring and Skip Ahead Insider testers.
One new test-build feature that sounds intriguing to me is adding links to Documents and Pictures folders in the Start menu by default. This is something I find an odd omission in Windows 10 right now: It's surprisingly hard to just find these folders quickly.
Microsoft has rolled over Disk Cleanup into Storage Settings as part of its ongoing work to consolidate Settings. It's also made some tweaks to Sound Settings with this build.
Microsoft also is adding autofill cards in web forms in Edge with this test build. It is updating the look and feel of the reading experience for EPUB, PDF and Reading View documents and pages in 17074. There's now a full-screen view for these document types as of this build.
In the list of general fixes and updates, Microsoft notes that this test build "includes mitigations to protect Insiders from the Spectre and Meltdown vulnerabilities."
There's also an interesting and somewhat cryptic reference to Windows 10 S in this build.
As recently discovered by some testers, those who've been using Windows 10 S for Insider testing have seen their PCs indicate that they are running "Windows 10 Pro in S Mode." (By the way, "Windows 10 Pro in S Mode" is the ungainly and official full Microsoft name for Windows 10 S.) Microsoft officials say "this change is by design" and that the company will have more to say on this as it approaches the rollout to mainstream users -- around April 2018 or so -- of Windows 10 Redstone 4.