Extend the canvas to make it 6x4 ratio with with a white area to one side? You can always cut the white part off later.
Another thing to bear in mind is that you might sometimes lose a little bit from all edges due to the printer allowing for a 'bleed' area, or something like that. It's a matter of millimetres, but it basically allows for any movement during the printing, so you don't get white edges if the print ends up slightly skewiff.