First let me say, I have no idea how many or what percentage of Canon 5D4’s this affects. It may be 2% it maybe be 20% or even more. If you shoot mostly in JPEG and do not aggressively edit, you may not even see it.

Here is how you do the test:

1. Use your hand or a piece of cardboard to over-expose in the left 1/3 part of the frame shooting RAW
2. Bring the file into Camera RAW
3. Bump up the exposure +1 or more (this makes it easier to see)
4. Raise shadows. It will look like the images below if you have it:

Side note, I did not see this in other cameras, including the Canon 5D3. It also does not happen in the middle, top, bottom or far right of the frame. Only on the left. If you do not over expose enough on the left, you may not see it as well. Here are some examples:

At +2 EvAt +1 EV0 EV (Very hard to see after it has been screen shotted and uploaded for web, much more visible in camera RAW)Defect always runs parallel to sensor.Defect doesn’t appear if there is not enough exposure in far left 1/3Also no defect on right side of camera.

Also no defect on right side of camera.