Who were the four caliphs appointed by Muhammad Ahmad al-Mahdi meant to represent?

Muhammad Ahmad al-Mahdi appointed four caliphs, or deputies, to serve as the living successors to the four earliest caliphs in Islamic history—Abu Bakr, Umar ibn al-Khattab, Uthman ibn Affan, and Ali ibn Abi Talib.12

By adopting this structure, al-Mahdi sought to model his government's institutions and nomenclature on those of primitive Islam. Of the four individuals he appointed to these positions, three were Sudanese, including his most trusted counselor and chief of staff, Abd Allah ibn Muhammad. The fourth appointee, Muhammad al-Mahdi ibn al-Sanusi, who led the Sanusiyyah order in the western desert, declined the invitation.1