I've never even SEEN a 4" Model 27. They are that scarce. You will probably have to pay a premium for its rarity. Actually, for that matter, I prefer the Model 27 3 1/2" - very classy.
The less expensive 4" Model 28 is relatively common, and let's face it, it's functionally and dimensionally the same. Here's one of mine, and it would be hard to tell it from a 27 - excellent blue, and factory target stocks.
John