MatchLive is organizing salsa-dancing mixers, walking tours and cooking classes in four major cities, including Chicago and Los Angeles.

FORBES: Sex and the City

Friends of the Earth said it had studied reports produced on air pollution in Scotland's four major cities last year and found most would fail to reach tough targets due to take effect in 2005 to cut levels of nitrogen dioxide, which is emitted by cars.

BBC: Valley 'most polluted area'

Several more of the best cities for women are located in the Northeast, including four cities in Connecticut and major hubs like Boston, New York and Baltimore.

FORBES: The Best-Paying Cities For Women 2013

RootMetrics recently compared 4G networks from the four major U.S. carriers in 27 cities.

CNN: Report: Verizon offers nation's fastest 4G service

China Central Television and other major media say four new people living in different cities in Jiangsu province are in critical condition with H7N9, a strain of bird flu that was previously unknown in humans.

BBC: China media: Concerns over bird flu

And to keep the playing field even, we limited the contenders to cities with at least 75 cumulative seasons in the four major sports leagues.

FORBES: Magazine Article