reconsider A = C, B = D as Subset of T ;
A2: ( A is open & B is open ) by Th2;
( A is closed & B is closed ) by Th3;
then ( A /\ B is open & A /\ B is closed ) by A2, TOPS_1:35, TOPS_1:38;
hence C /\ D is Element of OpenClosedSet T by Th4; :: thesis: verum