4. Three protons, initially separated by far distances, are brought together to sit in a straight line, with each proton $2.0 \times 10^{-6}$ m from the next (i.e., the first is at $x = -2.0 \times 10^{-6}$ m, the second is at $x = 0$ m, and the third is at $x = +2.0 \times 10^{-6}$ m). How much energy was needed to assemble these three charges?