repeat rewrite Zlength_map.  simpl.
repeat rewrite Zlength_app. repeat rewrite <- QuadByteByteList_ZLength.
reflexivity. Qed.
Definition SByte (q:SixteenByte) (v:val) : mpred :=
@data_at CompSpecs Tsh (Tarray tuchar 16 noattr) (SixteenByte2ValList q) v.

Lemma QuadByte2ValList_firstn4 q l:
firstn 4 (QuadByte2ValList q ++ l) = QuadByte2ValList q.