4 Plus is a subtheme of the City theme. While 4 Plus is alos a standalone theme, this subtheme focuses on a couple sets that are City specific. These sets featured larger pieces and minifigures than standard themes and the same as other 4 Plus sets.
The LEGO City theme is based on city life with the sets depicting city and emergency services, police, fire, airport, train, construction, and civilian services. The City theme proceeded LEGO Town which was replaced with LEGO World City in before it was rebranded as City in 2005.