Please see my latest paper that describes some advances in understanding the E8 to H4 rotation matrix

https://theoryofeverything.org/TOE/JGM/Unimodular-Rotation-of-E8-to-H4.pdf

Abstract: We introduce a unimodular Determinant=1 8×8 rotation matrix to produce four 4 dimensional copies of H4 600-cells from the 240 vertices of the Split Real Even E8 Lie group. Unimodularity in the rotation matrix provides for the preservation of the 8 dimensional volume after rotation, which is useful in the application of the matrix in various fields, from theoretical particle physics to 3D visualization algorithm optimization.