I'm looking into used PB-4 or PB-6 bellows for my Nikon system that I use to do amateur botanical shots. Currently I use a couple of macro lenses and some extenders. The price of each bellows is about the same. Will either one give me anything the other won't? If so, what?
PB-6 has more tolerances for more mounting rings, especially if one has to use old metal converters on medium and large format lenses. PB-4 can also mount those manual converters but for some tight angles may interfere with free movement of the bellows. PB-6 design is more accommodating.
That is the response I got when I asked a true expert, an expert who has been shooting macro photography for 37 years whom I met a local photography club meeting.