The 4xD print is 4" by 5.33". This is an aspect ratio (5.33 / 4) that is compatible with the aspect ratio of many digital cameras that use a 4:3 ratio or 4/3 = 1.33

The 4xD is 4 inches by 5.33 inches.  Dividing 5.33 / 4 = 1.33. By comparison, a 4x6 print is 6" / 4" = 1.5 aspect ratio, so the image must be cropped or, if it is printed uncropped to display the entire image, the print will have white bars on two sides.

Since a 4xD print has the same ratio as many cameras, no cropping is necessary for photos taken with those cameras. You will always see the complete photo when you print to a 4xD any photo taken by a digital camera that has a 4:3 capture ratio.

You can also set your camera phone to a 4:3 ratio and print to 4xD, so you will see the whole photo without any white lines on the sides.