4.

vigilante
治安维持会

[双语解释]

A person or group who takes the responsibility of a law enforcement officer into their won hands.
承担执法人员维持治安责任的个人和群体。

例句：

The subway vigilante in New York was in the media for weeks.
纽约地铁治安维持会几周来受到了媒体的关注。