Compressor 4 :: Cropping 1920x1080 Video With 4:3 Aspect Ratio

I have a 1920x1080 video that I want to crop to a 4:3 aspect ratio.

I'm working with compressor 3.  

When I set "crop original" to "4x3 1,33:1", I cannot manipulate the cropping. It is fixed to left=240, right=240, top=0 and bottom=0.

This results in 1440x1080 4x3, that's correct. But what I'd like to do is crop the top and bottom, too, while keeping the 4:3 aspect ratio.

How do i do that? 

When I set "crop original" to "custom" (I hope I translated that correctly), cropping left, top, right and bottom is possible. I manipulate the red rectangle in the preview window  and the cropping changes accordingly. Now how do I tell compressor to keep the 4:3 aspect ratio while manipulating/zooming with the red rectangle? I don't want to calculate the cropping, this is way too cumbersome.