Final answer:
The fourth important antigen other than A, B, O in the ABO blood group system is the Rh antigen, specifically the Rh D antigen.
Step-by-step explanation:
The fourth important antigen other than A, B, O in the ABO blood group system is the Rh antigen, specifically the Rh D antigen. The Rh antigen is part of the Rhesus (Rh) blood group system and is the most important antigen in this system. People who have the Rh D antigen are Rh-positive (Rh+), while those who do not have this antigen are Rh-negative (Rh-).