I don't have a four-inch, but I did have my SRH chopped to five inches after having it several years. Going from the original 7.5" down to 5, I lost less than 100 fps. My favorite load is 37.9 of 296 with a 240 XTP-Mag, and it went from ~2020 fps down to around 1940-1960. Heavier-bullet loads lost more than lighter-bullet loads.
Fwiw, the same 240 grain load (1950-ish from the 5" revolver) runs 2280-2290 from my 20" lever action. I got the rifle after having the revolver, and was surprised that the velocity increase wasn't more. Maybe that means that the difference going from 5" to 2 3/4" wouldn't be as great as we'd assume either...?
