Cropping tool for 4:3 or 16:9 images with Preview.app? Is there some trick to allow me to crop a screen shot in Apple's Preview.app to 4:3 or 16:9 dimensions? 

I did notice holding down shift causes a 1:1 rectangular selection, but I need other common ratios.

I am willing to accept a solution from other lightweight tools.
I do not use the Photos app for much, but I use it for this. I import the photo I want to crop. Click on the edit tool in the upper right and in the lower right you'll see "Aspect" with various options. You can crop to any of the desired ratios or customize a ratio. Once cropping is complete, I export the photo and delete it from Photos.

E.g., this makes it easy to crop a 4:3 digital photo to 16:9 for use as desktop.