So, just what is the difference between a 4 x 5 print and a 4 x 6 print, aside from the obvious – the size.
Portrait photographers of the past, before the 35mm and digital age came about, shot with large and medium format cameras. Medium format cameras shoot 120 or 220 film. The size of the negative for these is 645, 6×6 or 6×7. A 645 negative is 6cm x 4.5cm which is very close to the 8×10 format. The 6×6 negative size is square so it can easly be cropped for a vertical or horizontal 6 x 4.5. The 6×7 negative size is also very close to the 8 x 10 format. So, what you shoot with the medium format cameras is very close to the ideal crop for great looking images.
With the 35mm camera came 4 x 6 proofs, and prints. With digital cameras the proof, and print size, depends on the camera. Many point and shoot digital cameras actually have a 3.5 x 5 native print size. Digital SLR’s the professioanl photographers use have a 4 x 6 native print size. So, while for many years the professional portrait was delivered in a format very close to a 4 x 5, now many proofs are delivered as a 4 x 6.
The problem with a 4 x 6 proof is how it crops for enlargements. A 4 x 6 proof, if left full size is actually an 8 x 12, not an 8 x 10. A 4 x 5 proof has the same crop ratio as an 8 x 10. So, if you are shooting at 4 x 6 and shooting “tight” you loose alot of the images when a final 8 x 10 or 5 x 7 print is made. And, in my opinion the final image is not as strong when you loose that part of the image. And, when you shoot loose with 4 x 6 proofs, the proofs do not show the best image – you have to imagine how the final image will look as an 8 x 10, 5 x 7 or larger print.
I prefer to deliver proofs to my clients showing the final image. So, I shoot my images for the 4 x 5 crop. I use a crop screen in my camera. This crop screen has a line on each side that tells me where the 4 x 5 image is. Anything within those lines will be my image. Anything outside of those lines will be discarded in the proof and final image. So, what I shoot I get. My proof image will be my final image. There is no guessing what my final image will look like. I know it when I shoot it.
If you want to shoot a more professional format give the crop screen I use a try. I use the ProCrop. Give it a try!
Examples:
This is a 4×6 – no crop – as shot with my camera:

This is the same image as I shoot and process to a 4 x 5; this is also how an enlargement will look – 5 x 7, 8 x 10, etc:

Another example of a wedding portrait – 4 x 6 – no crop as shot with my camera:

Again, this is the same image as I shoot and process to a 4 x 5; this is also how an enlargement will look – 5 x 7, 8 x 10, etc:

I think you would agree the 4 x 5 images are stronger images than the 4 x 6 images.