Charge on plate X is Q1=Q and charge on plate Y is Q2=0
Charges on plate surfaces 1 and 4 are q1=q4=Q1+Q22=Q2
Charge on surface 2 is q2=Q1−Q22=Q2
and charge on surface 3 is q3=−q2=−Q2
The field at A is EA=E1+E4=(Q/2)2Aϵ0+(Q/2)2Aϵ0=Q2Aϵ0
As both 1 and 4 surfaces have same positive charge so fields at A and C are directed away from the surfaces and have equal magnitude.so are equal in magnitude but opposite in direction.
Thus, EA=Q2Aϵ0 (toward left) and EC=Q2Aϵ0 (toward right)
Now EB=E2+E3=Q/22Aϵ0+Q/22Aϵ0=Q2Aϵ0 (toward right) as filled is directed from positive to negative charge.