4.4 dates back to when I was still active (FYI I have not been active in the development community for 3+ years, kind if just popped in to check things out)
In general, a branch THAT old will not have any support and you likely won't find anyone bothering to code review any submissions. I'm not sure what the current state of the instructions for building for supported devices is.
No project has EVER had significant documentation on porting, because every device is different and the barriers you run into are different every time... It's something that is incredibly time consuming and you learn by doing. It requires general analysis and diagnostics/troubleshooting/problem solving skills. If there were a way to write a step-by-step porting guide - you would see far more devices supported at far higher levels of quality than what you actually see.
What device is this? I'm really surprised that you've found complete AOSP sources from the manufacturer. That's basically unheard of for anything other than Google devices and Sony's AOSP project. If you've merely found a kernel source code drop - congratulations, you've got around 1% of what you need.