If you're shooting at defense distances when at the "Range", there's no advantage to the 4" over the 3" bbl. in a "K" frame Smith. I'll define that as out to 15 yds in my experience. While I prefer the 4" M66 or M19 Smiths for carry here on the farm, for their better handling of recoil (read less punishing with moderately heavy loads), when carrying into town or Louisville, I go to a 3" M60 Smith for its lighter weight, by a considerable margin.
I'm not a fan of the full lugged "L" frame Smiths for carry where only a minimum of rounds are fired; but I'll concede that the extra weight makes for a more comfortable morning burning powder during extended range sessions. BTW, my carry loads with either, 3" M60 or 4" M66/18 average ~950-1000 fps. Enough for most any purpose, even putting down large stock on our farm, and more than enough for defensive use. These comprise 95% or more of my shooting. I rarely go to full house loads for any purpose, just too punishing.
With either bbl. choice, get a GOOD holster, one that rides well up on the belt, so you don't have the gun's hammer poking you in the ribs while seated in your car. I like an OWB, at 3:30 - 4;00 o'clock for that purpose. El Paso Saddlery makes some good ones, not cheap, but will last a lifetime of serious use. Look for the Tom Threepersons, Austin, or Askins Avenger types. And a GOOD gun belt of at least 1.5" width is necessary no matter which bbl. length you choose.
All things considered, were it one choice for the rest of my life, it'd be a 4" M66 Smith and I'd make wardrobe changes as necessary to conceal it when necessary. It's a gun you'll never sell... best regards, Rod