Proof
fun (F G X : Ens) (i : IN X (COMP F G)) =>
Comp_INTER_th1 F G X (COMP_pr2 F G X i).

Theorem COMP_th4 :
forall (F G X : Ens) (i : IN X (COMP F G)),
IN (PAIR (Comp_INTER F G X) (PR2 X)) G.