The work done required to put the four charges together at the corners of a square as shown in fig. is

See fig. The work done required is equal to the potential energy of the system. Thus W=U=14πε0q1q2r12+q2q3r23+q3q4r34+q4q1r41+q1q3r13+q2q4r24=14πε0−q2a−q2a−q22−q22+q2a2+q2a2 =14πε0−4q2a+2q2a2=q2−4πε0a[4−2]=−2⋅64πε0×q2a