Here is another 4" 27-2. In my experience, they are not rare, but they are less commonly encountered. I also seem to see more nickel 4" 27-2s than blued ones. As noted above, they became the "short" factory length for the 27 when the 3.5" guns were discontinued in 1979 or the early 1980s (I don't recall the specific dates, but I have a 3.5" that shipped in March 1979).
SN N831378 Shipped in May 1981:
All the best,