The original 'gang of four', Jiang Qing, Zhang Chunqiao, Yao Kenyan and Wang Hongwenan were prominent in orchestrating Chairman Mao's cultural revolution, 1966-76. So when four leading figures in the Labour Party set up the SDP in 1981, it was hardly surprising that the media dubbed them the 'gang of four'.
The reason they did so was that the January 1981 Wembley conference had committed the Labour Party to unilateral nuclear disarmament and withdrawal from the European Economic Community. They also believed that Labour had become too left-wing and had been infiltrated at constituency party level by Trotskyist factions whose views and behaviour they considered to be at odds with the Parliamentary Labour Party and Labour voters.
If Militant and the far left made Labour unelectable for 17 years, Momentum looks like doing much the same to today's Corbyn-McDonnell left-wing Labour Party.