Rafael Estrada just confirmed me on Twitter that he was the artist behind those System 4 covers: Evaristo el Punky, Khazzad-Dum and Cerius (System 4 release). He even retwited them among his followers and thanked me for the memories. I asked him if he remembers doing more cover artwork from that time. Anyway, I will credit Rafael Estrada through the Preservations Tool on ZXDB later today.
Regards