Seems to me they made it in 4" for a while. Shortening the barrel is possible but you end up with the writing on the barrel off center. I have seen some blued guns that have been shortened and re-blued. They looked prety good other than the off center writing. A stainless gun poses additional problems, ie re-attaching the front sight and blending it in.
You might be better off finding a 4" take off barrel.
If you are thinking of cutting a 6.5" barrel to a 4" model, I'd highly suggest selling the 6.5" model and picking up the 4" model. I've been looking for a -2 model 6.5" 610 for a while.. and they are hard to find.. Not to mention they are supposed to be coming available again as S&W has them in their catalog once again.