reconsider A = C, B = D as Subset of T ;
A1: ( 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 A1, TOPS_1:36, TOPS_1:37;
hence C \/ D is Element of OpenClosedSet T by Th4; :: thesis: verum