Is there an online printer that will print 4/3 large prints without cropping?

If you want to work around this, and you don't mind trimming the final print of two white (unprinted) borders, all you have to do is:

Open a new photo file that is 20x30.  Size your original so that its dimensions are 20 x 26.6666.  Copy the original and post it as a layer into the new file.  This way, the height will be the same size as the paper.  However, if you then center the print, you will have two white unprinted areas on either side of the image, which you can then cut down when you get the print or matt over them.

I used to do this when I shot 3:2 and wanted to get 8 x 10's printed.  Works fine, except you have to do a custom matt for it when you want to frame it.

-J