Explicit formulas for Grassmannian polylogarithms in weights 4 and 5

We explicitly reduce the Grassmannian polylogarithm in weight 4 and in weight 5 each to depth 2 iterated integrals. Furthermore, using this reduction in weight 4 we obtain an explicit, albeit complicated, form of the so-called 4-ratio, which gives an expression for the Borel class in continuous cohomology of ${\mathrm{GL}}_4\left(C\right)$ in terms of ${\mathrm{Li}}_4$.