This cool little cold-shoe-mounted gadget was designed to show the field of view of various focal length lenses on 35mm rangefinder cameras, but it's also useful for 4x5 or other large formats as long as you remember that 4x5 isn't as rectangular as 35mm (but 5x7 is fairly close). It has click stops for 35mm, 50mm, 85mm, 90mm, and 135mm, which roughly correspond to 135mm, 210mm, 350mm, 370mm, and 550mm on 4x5, and also a focus adjustment for fine-tuning the framing. I bought it thinking I'd use it with my 4x5, but it's never been out of my house and may as well find a new home.
It's in pretty good cosmetic condition as you can see. There's a bit of dust and a some internal haze, but neither interfere with its intended usage as a framing finder. The focus and focal length adjusters move smoothly. The case is included.
$55 shipped in the US, international shipping is more and depends on your location.