Open chapter 2 'Curving' from the introduction to general relativity "Exploring Black Holes" by Edwin Taylor and John Wheeler. On page 2-31 sample problem 3 is a description of the gravitational redshift. Just drop the M (it is the mass of the black hole in meters) because it cancels out for all r. Put a very large number for r(2), making it very far away from the black hole, and see what answer you get for the same problem where the light is emitted at the shell at 4M. Just so you won't be confused by 'mass in meters' the mass of our Sun in meters is 1477m making the Schwarzchild radius at 2M = 2954m. So the shell at 4M for a solar mass black hole would be 4(1477m)= 5908m.

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