Public Computing and the Next Gang-of-Four

"Every technology era has its four horsemen driving growth and innovation"

It appears to cherry pick solution. Mainframe computing in the 1960s and 1970s was dominated by IBM. IBM controlled over 50% of the market. The competition was first "the seven dwarves" and later five, also known as "BUNCH" (http://en.wikipedia.org/wiki/BUNCH).

What are the other three horsemen which could be compared to IBM?