When you do this, you are going from an image that has equal-length sides (a square) to an image that has a 4:3 aspect ratio. In order to do that you will have to cut out some of the image (e.g, resize to 400 x 400 and then cut 100), or else you will need to resize unequally so that what was a circle becomes an ellipse ("squished" from one side.)
There is no way to resize a square image to a 4:3 aspect ratio and keep all of the information in the same size proportions as the original.
