electric fields help

Direction of electric field is from A to B
Force on a positive charge is along the direction of the field, i.e along line AB
Charge on an electron is negative
So the force on electron is from B to A, opposite to the direction of motion.
There is a retarding force. The electron will decelerate.

Using loss in KE = gain in electric PE
The minimum KE needed for an electron to reach B from A is
loss in KE = qV = 6 eV
But the electron only has 4 eV, so it does not reach plate B.
It only has 2/3 of the energy required, so it reaches 2/3 along line AB. (v^2 = 2as, so s is proportional to kinetic energy)

When it returns to A, it covers 2/3 of the distance AB, so the work done by the resultant electric force on the electron is 2/3 x 6 = 4 eV, hence it has 4 eV of KE when it returns to A.