Hi universe_sound! The same problem applies to most of us because not only is the aspect-ratio for print different from the sensor but also the monitor or the TV on which we display the stuff. So cropping or down-sizing everywhere.
The solution? I normally don't take a picture without some room to breath around the major subject, so that cropping later is not a big issue. But when things are terribly tight and I don't want to lose a single pixel of the original, remember that the paper you print on can also be cropped
So for example print the 4:3 original full-sized and not cropped on the 4x6" paper, than you will have blank paper left&right. Just cut that away (down to 4x5.3") and everything is fine without losing any content!
Thomas (beware: Nikon-fanboy and moderator!) My Lens Reviews
, My Pictures
, My Photography Blog