I thought that the -4's came out in the late 80's, or close to that time.I have a Model 16-4 from 1972 and its mint with the box. I paid more than the asking price for yours. Hopefully you purchased it.
The K32 have grown on me, I have three pre-16, one 16 ND, one of each 16-2, 16-3 and 16-4. The popular barrel size is 6" as all are 6" except for one Pre that is a 4" and the 16-4 which is my 8 3/8".