The RZ Mamiyas are all part of that Medium format system. It sounds like you are re-inventing a bellows type (large format) camera. If you compare resolving power and other properties of LF to MF lenses, you might find it's economical to get an LF (bellows) camera and lens that will fill your needs. A Large Format camera will be larger than a PB-4, but will have all the movements, usually. There are some LF cameras in the crossover range, such as a 2x3". There many 'field-cameras' in these formats for sheets and often with roll-film backs (MF) such as Graflex and Linhof.

If you go to attach, say a digital or 35mm film camera to a LF camera, you can get an F-mount or appropriate Canon mount from an extension tube or a lens reversing mount.

Or sounds like you don't have the lens yet, you could get an LF lens and adapt a lensboard to fit the PB-4 - should be pretty easy, but you'd have to find one, like a 'press lens' that is small, so it will fit the front of that macro-bellows and allow for some movement.

To get the parts to adapt an RZ lens to the PB-4, probably best to adapt a lensmount from a broken camera. I'd check with camera repair and conversion (IR + UV) shops and check used departments at the biggies, like B+H, maybe Roberts, etc.