On 4 fenced acres of land that he can run you would not need to actually get him any additional exercise. If he is outside at all times and have full run of the property to patrol he will get enough exercise.  When the temperature is cold he will be very active. If it is hot he will sleep all day .. so on that kind of property don't worry about exercise unless you have him locked up in the house.  Again, if he lives outside on 4 acres he will get enough exercise.