4. Consider four point charges. Their location and charges in Cartesian coordinates are: A charge −2q located at (a,0,0), another charge −2q located at (−a,0,0), a 3rd charge −q located at (0,0,b), and finally a fourth charge +5q located at (0,0,−b). (a) What are the total charge of this distribution of charges? (b) What is the dipole moment of this distribution of charges? (c) Use the methods of "multipole expansion" (Griffiths section 3.4.1) to find a simple approximate formula for V(r,θ) (in spherical coordinates!) valid at points far from the origin. (Here, "simple" means only the first non-zero term is needed!) (d) Find a simple approximate expression for the electric field at points far from the origin. [Hint: the electric field is the negative gradient of the potential.]