If the male protagonist's code name in the novel was number four, then he might be an assassin who used to belong to an assassin organization. He might have excellent skills and keen insight to survive in the competitive assassin industry and escape the organization's constraints. After escaping from the organization, he might choose to start over or continue to work as an assassin or explore a broader path in life. Number Four might be a challenging character that would bring a lot of surprises and imagination to the readers.