According to Deadline, British actor Jimmy Akingbola has been cast as the villainous Baron Reiter in Arrow season 4.
Deadline describes Reiter's role as the "grizzled, hardened, and commanding" leader of an underground organization, who tries to recruit Oliver.
In comic books, Baron Reiter was better known as Baron Blitzkrieg, the leader of Shadowspire, a group with ties to South American drug cartels. Shadowspire also has connections to Vandal Savage, the main villain of the upcoming Arrow spin-off DC's Legends Of Tomorrow.
In DC continuity, Baron Blitzkrieg was also associated with the Nazis, but given Deadline's description and Arrow's contemporary setting, it seems very unlikely the Nazi connection will carry over to this version of the character.
Arrow season 4 premieres October 7, 2015.