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I see 6 inches are more frequent for sale which leads me to believe 6 inches are more common.

I see the exact opposite, I see the 4 inchers alot more than the 6 inchers. EDIT: as I was "working on my post" lol, I see ssgunnip beat me to the draw.
I have an NIB of each so no partiality to either side as far as that goes. I will admit I prefer the look of the 6 inchers, Especially the first years with the Dual Tone Finish, in the white muzzle, and Full-Checkered grips with old style medallions.

 

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Well i have a 4" am looking to get a 6" .
Judge ,does this one have a dual tone finish or is the lighting just wrong,what about the polished muzzle ? this one does not have it ,is that correct for this late '61' model.

I am not Judge and Judge is much more knowledgeable than me , but I do know a little about these. These later ones do not have the Dual Tone finish or "polished muzzle". This one does not have Dual Tone.
This gun makes me scratch my head with the SN#. My "very Late 1960/Early61" NIB 4 inch model was one of the last shipped according to the 3rd edition of TBOCF. And it is the only one which I ever seen shipped in a Trooper box. I had heard rumors of this but never verified until I got this gun. There is no doubt the box is original, there are about 4 or 5 things which verify this, stamps on box, box numbered, stamps on paperwork inside, and original test target. Mine is very very close to that 28000 range "cutoff" according to the book. This gun is 1600/+ higher SN# than mine. I know you can never say never with Colt and they may have had some parts left they didn't know about. Finish does LOOK 100% original to me from what I can tell of the pics, but I took this seller off of my "trustworthy" list along time ago, because I have seen him pass A BUNCH of fake stuff. And I mean a Bunch of fake "NIB packages" including the best I can remember, a couple of refinished guns as original.

 

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Below you see #30214 - which exhibits enough honest use to suggest the "cutoff" in serial numbers was higher than the previously cited 28000 range.

Jerry

Yes I think that is now proven . Thanks for sharing. I guess they missed that last run of 357s when they wrote the book, and It's still in 1961 which makes sense. This isn't surprising and would be easy for them to miss since in the later 357's, the Troopers greatly outnumbered 357's in that serial number range.
Some things don't make sense at all with Colt, just one example I've seen: my NIB Army Special in 32-20 which dates to 1933 in the official police serial range, and according to the original Railway Express receipt was shipped March of 1937, which I took a gamble on and glad I did when I found out they did make that run.