@kvbarkley wrote:You assume incorrectly. The sensor is the sensor is the sensor. Anything except the full-size 4:3 is a crop. In fact, if you shoot RAW you can usually get the full 4:3 image back.
Actually, the native aspect ratio of a 5D4 is 3:2, not 4:3. It's late at night, so you get a pass. 
@jamespotter wrote:Hi am looking to take some cinematic style pictures and was looking into shooting with the 16:9 aspect ratio settings but had some questions about it. I am assuming using that ratio you get higher pixel count than by shooting full and cropping? So mainly does it make sense to shoot that way vs just cropping it later? Also it seems like the only way to actually capture shots in that aspect ratio is to use the live view mode?
Yes, your assumption is incorrect. The highest resolution images that the camera will capture will be at the 3:2 ratio. Any other ratio is a cropped version of the native 3:2 image ratio.
Every image your camera captures will be at a 3:2 ratio. Your 16:9 aspect ratio is created by cutting off a slice at the top and bottom of the original 3:2 captured image. A square image cuts off slices from each side.
The ratio setting only makes a difference if you are saving your images as JPEG, which crops the image to your aspect ratio setting. A RAW file stores the setting in EXIF data, and displays it to you at your aspect ratio setting, but the RAW image is still a full 3:2 ratio.
I suggest that you learn to shoot as RAW, and perform the cropping in post processing when you create JPEGs. Doing it in the camera will always center the crop, which may or may not be the best choice.
For example, I like to shoot stills from a tripod with wide angle lenses. I want to keep the camera level which puts the horizon in the middle of the shot. Sometimes I want to see more of the sky, with the horizon situated lower in the frame.
I can make this adjustment in post, but not in the camera.
