Bill,
If you have procedures for welidng 8620 or 4140 in wrought material, it will generally work for castings also. The large grain structure of castings that forms during solidification is refined when they are austenized, water quenched and tempered. Properly melted, poured and heat treated castings can be easier to weld than wrought material in some cases, as you won't run into lammelar tearing, or directional property problems. You can run into other problems though, like micro-shring, hot tears, or gas. Proper NDT can elimiate these problems though, if they will cause a problem for the end user. We don't make a 4140, but we do make a 8620, and most of your variables will depend on the temper of the base material. Generally, we use 300F preheat min. Weld metal from 80-120 ksi, and PWHT is usually mandatory. We temper from 1050-1150F, but you generally want to temper at 25-50F min. below the previous tempering temperature.
Good luck with your project.
G Roberts