There was nothing changed in the 4th Model from 1895 to 1909! The steel, the design, the parts remain exactly the same. Standard steel tensile strengths in excess of 30,000 psi were available shortly after the Civil War and were more than 3 times stronger than peak pressures produced by shooting standard 38 S&W. By the late 1800s, that strength was increased to 50,000 psi, making these old guns even better.
If a qualified gunsmith said it is ready to go, it will shoot "evil" smokeless powder just fine. I have shot thousands of rounds of 38 S&W through the years in my top-break S&Ws without issues. I found it absolutely safe to shoot standard modern loading. If one compares full load BP to Remington/Winchester 38 S&W, you will find significantly less felt recoil with smokeless. One last comment is the question if any ammunition manufacturer would make an almost obsolete caliber that was not thoroughly researched, tested, and loaded light knowing that much of the ammo will be shot in top-break revolvers in today's litigious world? They do not even warn top-break users about using their ammo on the box!
Can a part break on these old guns, certainly, but even a small spring can break, but not because of shooting smokeless. Everyone can do what they want, but the facts do not support their decisions to shoot only BP and never will, plus cleaning the outside is not good enough in humid climates, which makes a thorough cleaning necessary after shooting.
Here is some of my testing data on modern 38 S&W factory ammunition as compared to BP from both pre- and post-1900.
38 S&W 145RN Factory PPU . . . . . . . . . . . . . . . . . . . . . 599 fps
38 S&W 145RN Factory Winchester . . . . . . . . . . . . . . . . 615 fps
38 S&W 145RN Factory BP Winchester Pre-1900 . . . . . . .685 fps
38 S&W 145RN Factory BP Winchester Post-1900 . . . . . . 687 fps
38 S&W 145RN Factory BP Remington UMC . . . . . . . . . . 740 fps