unfold S4 in |- *...
intros...
assert (S B D A = S A B D)...
assert (S B C D = - S C B D)...
assert (S4 A B C D = S A B D - S C B D)...
rewrite <- H in H1.
rewrite H0.
unfold S4 in H1.
rewrite H1 in |- *.
ring.
Qed.
Hint Resolve S4_2: Geom.

Lemma S4_3 : forall A B C D : Point, S4 A B C D = S4 C D A B.
Proof with try solve [ Geometry | congruence ].