All you have to do is revert the process shown in the video (where the author uses a cheat sheet copied from a MR thread without any credits given).
Make the root of the filesystem read/write:
The K4 (and K5) are "dual boot" machines - your booted into the "diags" system (stored on mmcblk0p2)
So you have to mount the "main" system (stored on mmcblk0p1) to make changes to it.
The video tells you to use /mnt/mmc-base - So I will assume it is still available (I don't have a K4 to check) here.
mount /dev/mmcblk0p1 /mnt/mmc-base
And in reverse order - - -
Those directions created a symbolic link:
ln -sfn /mnt/us/screen_saver /mnt/base-mmc/amazon/screen_saver/600x800
So remove it:
rm -fr /mnt/base-mmc/amazon/screen_saver/600x800
Those directions had you save the originals by moving them to a new name:
mv /mnt/base-mmc/opt/amazon/screen_saver/600x800 /mnt/base-mmc/opt/amazon/screen_saver/600x800.old
So move it back now:
mv /mnt/base-mmc/opt/amazon/screen_saver/600x800.old /mnt/base-mmc/opt/amazon/screen_saver/600x800
Those directions had you make a screensaver directory -
Get rid of that by removing it and all of its contents:
rm -fr /mnt/us/screensaver
To accomplish the above, you had to make two system changes:
you mounted the "main" file system;
you made the "diags" file system read/write;
Both of those changes will be reset when you restart you kindle.
Now all you have to do is watch your favorite video again at:
only when the author gets to the part of cut&pasting commands that he does not have a clue about - instead use the above bold font commands.
The video references /mnt/us/screensavers in the "DIAGS" file system
- which may be an error unless the "DIAGS" mode has mmcblk0p4 mounted at the same point that the "MAIN" mode has it mounted.
(mmcblk0p4 is the device that holds the FAT-32 file system that the user sees as the "USB storage" over the cable when in normal operation.)
PS: It is not a "propriatary USB cable".
You can check if the above is so by entering the command:
without any arguments and read if/where /dev/mmcblk0p4 is mounted (and if it is mounted read/write).