There seem to be more Pre-17 K-22s in my area (Midwest) than Pre-18, and especially 5-screw versions.
I would guess that all things being equal, a pre-18, Combat Masterpiece, 4", 5-screw, would sell for $200 more than a comparable pre-18 5-screw 6" K-22. The difference might be even greater, not sure.
What is your area?? The Midwest is a huge area and you do not note where you live in your bio. Where one resides can have something to do with prices, but not value, especially since one has not had to buy their guns locally for years. I will argue that better prices can often be found online and if the buyer is aware of what is for sale, askes questions, requests additional photos, you will get what you want for less than gun shops more often than not.
Supica/Nahas actually show lessor condition Combat Masterpiece revolvers with lower value than comparable 6" guns. My guess is that demand for K22 short barreled guns is less than 6" guns, since the main purpose of this model is target shooting. As for more 5 screw Masterpiece revolvers than Combat Masterpiece revolvers, the 6" model started in 1946 while 4" model did not start until 1949. Also, it is surmised by some that there are less 4" CMs out there because demand was less than the Masterpiece revolvers throughout CMs production run.