Scope choice depends mostly on intended use, and to some extent, the rifle. Given the limitations of a 9mm, a 4X is as high as you need to go.... a fixed 2.5X would probably be a better choice.
If I were to scope a 9mm Carbine, I'd look at a 1-3X or 1-4X variable; a fixed 4X is way too much for a close and moving target, but would be okay for strictly target use.