The 4B-1 I had came with a 6x4.5 mask. Drop it onto the film plane before loading the film. The two film counter window on the back will allow you to tell when to stop advancing the frame. To compose, the focusing screen has two guidelines to aid 6x4.5 composition.
When I bought the camera a few years back the seller recommended that I avoid using the mask... something about the quality of the construction, might scratch the film or something. Not sure about QC now.