Since it could be an Old Army #2, I'll ask if the cylinder stop is in the bottom frame or is it on the top of the frame? First Model 1 1/2's had the stop in the bottom strap.
A Model #2 with a factory 4" barrel is quite scarce and usually are expensive. Fakers figured that out and cut 5" & 6" barrels to 4". To authenticate the barrel length, look at the S&W address on the rib. If the address is centered then, most likely it is legit.