Using Kuperberg's web calculus (1996), and following Elias and Libedinsky, we describe a "light leaves" algorithm to construct a basis of morphisms between arbitrary tensor products of fundamental representations for sp 4 (and the associated quantum group). Our argument has very little dependence on the base field. As a result, we prove that when OE2 q ¤ 0, the Karoubi envelope of the C 2 web category is equivalent to the category of tilting modules for the divided powers quantum group U A q .sp 4 /.