Both of these are -4s. The 4" is a round butt 686+, serial prefix CBN. The 6" is a square butt 6 shot, serial prefix BPW. You can see they have different finishes on the trigger and hammer and a different style cylinder release as well. Also, the 6" has no white outline on the rear sight, the 4" does. Found the 4" on consignment at a local shop recently. The 6" I bought new in the mid 90's. I replaced the target stocks that came on it with the Hogues. The 4" came with them. I've often said if I could have only one gun, it would be a 4" 686+.