  We elucidate how integrable lattice models described by Costello's 4d
Chern-Simons theory can be realized via a stack of D4-branes ending on an
NS5-brane in type IIA string theory, with D0-branes on the D4-brane worldvolume
sourcing a meromorphic RR 1-form, and fundamental strings forming the lattice.
This provides us with a nonperturbative integration cycle for the 4d
Chern-Simons theory, and by applying T- and S-duality, we show how the
R-matrix, the Yang-Baxter equation and the Yangian can be categorified, that
is, obtained via the Hilbert space of a 6d gauge theory.
