The difference between Mesinger 3 and 4 eluded me for a long time, i only have access to No.3's but the conclutions I've come up with is that the No.4 has about an inch wider seat pan. It is kind of hard to spot on pictures if you don't know where to look and I think there is a mixup in some cases where some 3's has the 4's pan as well as the other way around. It might be so that the No 4 is also about 3/4 inch longer in the snout, I have seen longer tension springs than those I have. The front pommel can be of 2 versions, 4 or 6 rivets, but I think that was fitted on both No 3 or 4 seats. I belive you could order either of the seats from the factory if you knew how, but I think most bikes where delivered with the No.3. So, what you prefer might depend on the width of your own bottom! Pictures of reproductions of the different seats can be found here http://www.oldtimerservice.riders.pl/part1.htm