Surface Pro 4 Firmware Update (v1.0.420.0) "enhances stability with Surface Pro 4 Type Cover" for the Surface Pro 4, while Surface Base Firmware Update (v.1.1.421.0) "improves touchpad and keyboard experience" for Surface Book owners.
The updates are being rolled out in stages, so Windows 10 might not have been told you about them yet. If that's the case, head over to Windows Update where they will apear as "System Firmware/Hardware Update – 12/17/2015".
According to owners in one Reddit thread, Microsoft's latest 2-in-1 has been plagued by Type Cover-related issues that cause Windows 10 to boot slower, random trackpad crashes when waking from sleep and screen flickering with the brightness turned down.
Some Surface Pro 4 users reported faster boot times with the keyboard attached and smoother two-fingered scrolling using the trackpad after applying the update.
Microsoft recently apologised to Surface Pro 4 and Surface Book early adopters who have had to contend with minor and major issues affecting usability since the devices went on sale.
Via Digital Trends