GKZ-system of the 2-loop self energy with 4 propagators Tai-Fu Feng, Hai-Bin Zhang, Yan-Qing Dong and Yang Zhou Eur. Phys. J. C, 83 4 (2023) 314 Published online: 21 April 2023 DOI: 10.1140/epjc/s10052-023-11438-6