First we calculate the Electric Force
F = qE = 1.6E-19 * 4 E+4 = 6.4 E -15 N
This Force is equal to mass*acceleration
F = m*a
Mass of the electron = 9.1 E-31 Kg
Mass of the proton = 1.67 E -27 Kg
Acceleration of electron = 6.4 E -15 / 9.1 E -31 = 7.033 E 15 [m/s**2]
Acceleration of proton = 6.4 E -15 / 1.67 E -27 = 3.832 E 12 [m/s**2]