Primordial trispectrum ($T({\mathbf{k}}_1,{\mathbf{k}}_2,{\mathbf{k}}_3,{\mathbf{k}}_4)$)

The primordial trispectrum is the connected four-point function with its momentum-conserving delta function removed. Exchange diagrams split naturally into $s$, $t$, and $u$ channels according to which pair of external momenta meets at one cubic vertex.