Canon - RF800mm F11 IS STM Telephoto Lens for EOS R-Series Cameras - Black

Yes. The EOS R6 uses the RF lens mount that this lens has. I haven't done much in wildlife photography. I can use 1/200 to 1/300 shutter with 400-800 iso for the most part in daytime for what I shoot. It is very good for reasonably still wildlife shots like deer, heron and perched birds. I'm not very skilled in tracking birds in flight, which I imagine a faster shutter would be desired. Would it be good for your purpose? The RF 600mm F/4L $12,999 or the RF 100-500mm f/4.5-7.1L $2,799 would be better pro level options. But, if willing to push iso levels, the extra reach and cost savings may be just the right balance to be good.

Yes to autofocus. Remember this is an "RF" mount lens, not "EF". And for the money, it is a great lens.

The RF800mm F11 lens is NOT compatible with the 4000D. The EOS 4000D uses "EF" mount lenses. The Canon RF800mm F11 lens requires AN "RF" mount. There is no adapter available at this time.

Unfortunately - no. This is an RF lens for mirrorless cameras. I have a 90D. Have you tried a Sigma Lens?

This is not easy to answer. Range is not a way to compare lenses as it is the field of view that gives the perception of magnification. Any 800mm lens on a full frame camera has a field of view of about 2.58 x 1.72 degrees. At 30 feet away, an area of 1 ft 4 in x 10 in will fill the frame, say a squirrel will fill the frame. At 240,000 miles away, an area of about 10,000 x 7,000 miles will fill the frame, say the moon fills most of the frame. Photographers will sometimes refer to the narrowing field of view as the reach of the lens, or how well it makes distant things look bigger. The more expensive lens can provide a better image of a small part of the frame, but will not change what fills the frame. This 800mm has a great reach for the buck, but would be better for wildlife mostly at rest. Birds in flight, with this narrow field of view, make it difficult to track and focus.