When 4x6, 4x3 cropping necessary?

Question

Why does iPhoto's constrain pull-down offer two 4x3 options – 4x3 (DVD) and 4x3 (Book)? Either seems to lead to the same cropping results...or am I missing something?
Our camera (Canon PowerShot G6) seems to shoot in 4x3, though the goofy G6 owners manual seems to offer no confirmation of that. So is it correct we do not need to crop our apparently 4x3 images at all (unless we wish to select a smaller portion of the image) when we prepare an iPhoto book?
Now when it comes to ordering 4x6 prints through apple ("Kodak Print Service") I've learned the hard way that I should have constrain/cropped to 4x6 to have SOME control over the cropping done by the print-maker. Everything outside the 4x6 ratio seemed to have been chopped off the bottom everytime, but not inspecting all 140 prints, perhaps it was much more random.
Next time I will first crop to 4x6. But now I wonder if there's information about the exact amount of image that will be cropped on each side (since only borderless prints are offered) at that particular print-making source – when 4x6 images are submitted. Is it always the same? Or does it vary in some random fashion? Or where can I find more discussion of this particular detail?

Answer 1

mulerider:
In iPhoto's Edit window there's a Constrain value 4X6 Postcard. You should use that. If you're working with a portrait oriented picture, go back to the Constrain menu and select Portrait at the bottom. You can then use the cursor to select whatever part of the image you want to print. Then click on the Crop button.