I think how each of the 4 yonko could have a connection to their corresponding themes could be:
White Horse Man: Luffy (Conquest)- Luffy doesn't actively Conqueror other nations but rather saves and frees them
Red Horse Man: Shanks (War)- The most diplomatic of the original yonko, but people also speculate that Shanks is based on Tyr, the God of War from Norse mythology.
Black Horse Man: Teach (Famine)- Teach is power hungry
Pale Horse Man: Buggy (Death)- Buggy has the best luck and always survives?