The Linux Kernel version that Android is based on is called as Kernel Version. It's the core of the operating system which handles requests to and from the hardware, memory and process management and all the low-level stuff that is necessary for Android to be able to run.
In the practical world, the kernel is the softer, usually edible part of a nut, or seed and is contained within its hard shell. When talking about smartphones, tablets, and even computers, the kernel isn’t that much different… though I wouldn’t recommend you eat it. Think of it like popcorn. What Is A Kernel?
Actually, both of any versions of Linux kernel, varies from 3.x and 4.x can be on Android 9 Pie, without any major problems be made. Still, it is better that any device that comes out with Android 9 Pie out-of the box, to have a copy of Linux version 4.x, so those Android devices can function at their best.
Starting this year with smartphones which ship with Android Oreo, Google is requiring that all SoCs productized in 2017 must launch with kernel 4.4 or newer. Currently existing devices that are upgraded to Android Oreo are only required to run kernel version 3.18 or newer, and will not have to be upgraded to support Project Treble.