Megapixels
Thank God for megapixels! The 5D Mark IV has incredible density which allows you to crop down and still get a decent image. Here are some perspectives and one highly cropped photo that I probably wouldn't call usable.
This is the original photo at 200mm.
I was able to get the 2x Extender on which brought the photo to 400mm.
This is the first crop where I changed the orientation.
This is cropped WAAAYYYY down. Probably couldn't print it and the detail is not great at this level. I'd call this one unusable.