destruct B as (((q0, q1), q2), q3). simpl.
rewrite SixteenByte2ValList_char, app_nil_r. trivial.
Qed.

Lemma QuadByteValList_ZLength q: 4 = Zlength (QuadByte2ValList q).
destruct q as (((q0, q1), q2), q3). simpl. reflexivity. Qed.