Correct 4 miles, as most calibers will not shoot any farther.
If you are talking about shooting accurately then it is more about the shooter and very little about the equipment. Then there is your meaning of accurate. Is it a 1 inch group or a 30" group. Shooting at targets of a known distance or shooting at live things at unknown distance and various atmospheric conditions. Are you shooting a 22 long rifle or a 50 bmg?
The magnification of a scope does not relate directly to how far you can shoot with it. Much of THAT is the rifle, the caliber, and the shooter. And hey- I can get about 5 miles out of a 30-06, guaranteed to hit the Earth every time.
Higher magnification generally means smaller field of view, and less light that makes it through the scope. Two scopes can have same power, but one be MUCH sharper and brighter than other (quality of lenses, compound that sticks one lens to the next, etc. ) I generally do not go over 4 power when deer hunting, but for prairie dogs use a scope that will let you see the rings of Saturn. For a moving deer, I would not be able to FIND the deer in that tiny field of view of the high powered varmint scope.
Sort of like asking "I have a V-8. How fast will it go?" Depends- it is the V-8 in my Ford pickup, or the V-8 in my 69 Z-28 Camaro?