Speaking of cropping 4x5 to have either that ratio or anything in between... one can always take two exposure on 6x12cm shifted up/down. With a 1cm overlap that nets a roughly 10x12cm image (actual size closer to 92x112cm) but it's even squarer than 4x5. Of course, you'll have to stitch them in PS and crop as you desire. It's just another option.