reserve x,x1,x2,x3,x4,y,y1,y2,y3,y4,z,z1,z2,z2,z4 for object;
reserve X,X1,X2,X3,X4,Y for set;

theorem
  proj2_4(X /\ Y) c= proj2_4 X /\ proj2_4 Y
proof
  proj2_4(X /\ Y) c= proj2_4 X & proj2_4(X /\ Y) c= proj2_4 Y
    by Th17,XBOOLE_1:17;
 hence thesis by XBOOLE_1:19;
end;
