Clasped web bases from hourglass plabic graphs

G.-Pechenik-Pfannerer-Striker-Swanson applied hourglass plabic graphs to construct web bases for spaces of tensor invariants of fundamental representations of $U_q({\mathfrak{s}\mathfrak{l}}_4)$, extending Kuperberg’s celebrated basis for $U_q({\mathfrak{s}\mathfrak{l}}_3)$. We give several combinatorial characterizations of basis webs in the kernel of the projection to invariants in a tensor product of arbitrary (type $1$) irreducibles. We apply this to show that the nonzero images of basis webs form a basis (a property shared with Lusztig’s dual canonical basis) yielding distinguished clasped web bases for each such tensor product.