The two aspect ratios are different shapes. When you change aspect ratios you have to choose between cutting off pieces of your image or leaving the white space (letter boxing). You can't avoid one or the other unless you stretch the image.
That said it is possible to crop in bulk using the Quick Develop panel in the Library module. Use Grid view and select all of the images you wish to change and crop them to 4:3. You will lose part of the image on each side (left and right).