This is comical. PEOPLE: I took ONE PHOTO, the SAME PHOTO, and resized it twice in software to show the exact difference in megapixels and cropping ability you would get between 20mp and 24mp. IT'S THE SAME PHOTO. Better color and shadows? IT'S THE SAME PHOTO.
I opened my laptop and found some random bird photo. I resized it in software to make it exactly 20 megapixels. I then took the exact same photo and resized it to be exactly 24 megapixels. I then put both images, side-by-side on the screen, and used the image viewer to ensure each was being viewed at "100%". I then took a screenshot. The "composition" may be different because of where I happened to scroll the image to position it on my screen. Zero relevance. Each photo, side-by-side, viewed at 100%, represents the exact difference a photographer would get had they photographed that bird (1) with a 20 megapixel camera and (2) with a 24 megapixel camera. The purpose of the comparison is so that someone can look at the size of the eye of the birds, or look at the size of the beak of the bird, or any other part of the bird, and THAT IS THE SIZE DIFFERENCE BETWEEN 20MP AND 24MP. That's it. THERE IS NO MORE CROPPING ABILITY THAN WHAT YOU SEE IN THOSE TWO BIRD PHOTOS. I don't know how to further make it understandable. That is the cropping ability. I have already done it for you. It is already cropped and being viewed at 100% for each of the two megapixel sizes. The bird photo on the right is the extra reach you get from the extra 4 megapixels. The increase in the size of the eye, that you see in those two photos, IS ALL YOU GET with an extra four megapixels. The increase in the size of the beak is ALL YOU GET. That's it. It may be frustrating to realize how little it is, but THAT IS IT. If you had an editor that wanted a large photo of the bird, and you owned a 20 megapixel camera, and a photographer who was your competitor had a 24 megapixel camera, and the editor said they wanted a bigger, higher resolution photo of the bird, the difference that editor would get between you and your competitor is the difference between the bird on the left and the bird on the right. That is the sum total of size increase between the two cameras.
As far as the "skilled photographer" comment, stop being patronizing. I'm a professional commercial photographer and have my photos in countless magazines. This is the first time I've mentioned that on this forum and hopefully the last. Personally I don't find it relevant to having fun on these forums, so I don't say it. But I don't need the attitude. I literally opened my laptop and grabbed the first random photo that popped up on the screen (I was testing a telephoto lens if you must know) and used it as an example. Your statement about 4 megapixels being 'substantial" in the hands of a skilled photographer is, quite honestly, preposterous.
To sum up, the difference between 24 megapixels and 20 megapixels in practicality is very, very small. It just is. It can easily be seen by looking at the two bird photos. That's the difference. If someone wants to look at the two bird photos and say they see a very significant difference in the size of the eye, or the beak, great. Go for it. I don't see such a material difference that it would possibly affect anything. If any of my clients saw this discussion about the size of the bird in each phto, they would laugh and have no idea what we were talking about. They'd probably see no difference whatsoever.