The 4566, 4506, etc mainspring is the "long" mainspring. Same as the 39, 59, 5906, etc.
The 4516 uses the "short" mainspring like the 6906.
They are not avialable from S&W in varying weights, only the "long" or the "short".
The custom shop (performance center) might have some other choices.
If you go with a lighter spring, you'll find it's a huger improvement over the standard DA pull. I have never had any reliability issues with mine after cutting two coils off. (this brought the weight about on par with the Wolff 17 pound spring)