X server crash

We have a custom board with 4GB memory.
The output of HDMI and eDP port are both 3840x1080. (each split to 2x ports so there would be connected to 4 1920x1080 monitors).
When we play video for long time, there would be X server crash.
Then we found this thread in forum:
As #18 solution,
We modify boot arguments in /etc/extlinux/extlinux.conf:
add ‘video=tegrafb’ and modify ‘mem=’ changed from ‘mem=2015M@2048M’ to ‘mem=1862M@2048M’

This seems to fix the issue, but we have some questions:

  1. what does “tegra_fbmem=32899072@0xad012000” mean in #18 solution.
    Since the TK1 board is with 2GB memory,
    How should we determine the value for our board with 4GB memory?
  2. What is your advise on this fix, will there be any negative impact?