In every Windows 10 feature update development cycle there is a small window between when a stable build is released, one that is likely the near final build of that update, and when Microsoft begins releasing new pre-release builds for the next feature update for Windows 10.
If you are like me, once Windows 10 Redstone 3 Build 16299.15 was released and ushered in the focus on cumulative updates for the next feature update then you likely started to upgrade/install it on more of your devices. That means you probably moved those devices into the Windows Insider Fast Ring to grab 16299.15.
However, most of us do not run early development fast ring builds on these same devices so that means you need to take action to prevent joining the Redstone 4 Fast Ring.
To keep that from happening you need to adjust your Windows Insiders settings over the next 24 hours or so because it appears it is time to get off the Fast Ring train unless you want to shift the current Fast Ring for Redstone 3 over to the pre-release channel for Redstone 4 - the feature update that is expected in March 2018.
You have a few options at this point to while this window is open:
Move to Windows Insider Slow Ring
Microsoft will not be releasing any Redstone 4 builds to the slow ring anytime soon so this is a safe holding place for now to stay on Redstone 3 and receive any possible last minute cumulative updates when they arrive for slow ring devices.
Move to Windows Insider Release Preview Ring
Under Release Preview no builds are pushed to your device and the focus is on receiving update versions of various inbox apps. Short of leaving the Windows Insider program on a device, this is a very safe place to hold tight and not inadvertently get a Redstone 4 build.
Leave Windows Insider Program
This is a per device option and we are not talking about leaving the program permanently. However, this is a special time in the Windows Insider program when you can remove a device from the program and not be forced to do a clean install of the OS.
If your device had a digital entitlement to Windows 10 it will pick that up as its genuine status and of course the system will only receive maintenance updates as they are pushed out each month on Patch Tuesday.
Skip Ahead Channel
If you have a device that is in the Skip Ahead branch for Redstone 4 I recommend not changing anything on that device as I suspect you want to be there. Eventually Microsoft is going to transition those devices to the normal fast ring for Redstone 4 development but since leaving that branch means you loose it for good, I would wait just in case there is something else coming to that branch.
For my own setup - I moved four devices to 16299.15 to join a Surface Book in the fast ring and an HP Spectre x360 (2015 model) on Skip Ahead for Redstone 4.
As we move to the next development cycle I will leave the Spectre in the Fast Ring and everything else will remain in the Semi-Annual Channel (Targeted) for the foreseeable future.
What are you plans for the transition between Redstone 3 and Redstone 4?