Potential at a point A is 3 volt and at a point B is 7 volt, an electron is moving towards A from B.

Potential at A is 3 volts. Potential at B is 7 volt. 

Electric field should be in the direction from B to A. So, the force on the electron will be towards B. 

For electron to go from B to A, work must be done. So, it must have some KE  at B to reach A. 

Minimum kinetic energy must be required at B to overcome the electrostatic force to move from B to A.

$\Rightarrow {\mathrm{W}}_{\min }=∆{\mathrm{K}}_{\min }=\left(7-3\right) \mathrm{eV}=4 \mathrm{eV}$