Differences between the PB-4 and PB-6 bellows for my Nikon camera?

I agree that the PB-4 is sometime narrow and cannot be used with big cameras with integrated grip (professional SLR / DSLR).

I found as well this interesting detailed article which explains the usage of modern cameras (like DSLR) to use with the Nikon bellows especially with the PB-4.

At the end of the page, there is a section "The PB-4 compared to the other Nikon bellows units" which give most of the differences.

To summarize the difference with PB-6:

• PB-4 could be too narrow for mounting some cameras, I personally was able to mount D300, D500 and D600 without issue (sometimes I had to switch to vertical mount and rotate the direction after the camera is mounted)
• PB-4 have the tilt & shift feature that the PB-6 does not have
• The PB-4 knobs are in metal as the PB-6 ones are in plastic
• PB-6 can be extended longer with an extension (PB-6E) where you can reach a ratio of 22.3:1 with a 20mm f/2.8 reversed (distance from the front lens: 34.4 mm) The PB-4 can "only" reach 12:1 but such a ratio is already difficult to manage (and the PB-6 alone has the same ratio)
• PB-6 has an automatic aperture cable release port when you need an extra ring BR-4 with the PB-4.

For the build quality I cannot say too much on the PB-6 as I did not tested it, but even with its plastic knobs, it looks quite sturdy. Note that there is also only one rail in the middle as the PB-4 has 4 rails in parallel: I do not know if there are any advantages.

On my side I found a good second-hand PB-4 in a very decent condition and I would recommend it if the reproduction ratio is not a strong goal (PB-6 + PB-6E) and if the price is also decent. The tilt & shift feature is also interesting but quite difficult to master as well (live view can be helpful).