Industry 4 refers to the fourth industrial revolution where machines interacts with softwares and work processes to ensure a production process with no (or less) interruptions.
Industry 4.0 - History and background
Industry 4.0 has its background in Germany and the word itself was first presented by Angela Merkel when she initiated a national initiative to take the German industry to the next level. That was in 2012.
What about Industry 1, 2 and 3?
The very first industrial revolution took place in the late 18th century, when manufacturing began to go from manual labor to mechanization. Steam engines and other water-powered engines revolutionized industry and fundamentally changed production and labor.
The second industrial revolution took place in the early 20th century, when industry began to electrify. Electricity enabled mass production and a much more efficient production and industry.
The next paradigm shift in industry took place in the late 1950s when electronic technology and later computer technology entered the scene. Digitization began and gradually changed all production right through. Analog and mechanical technology declined and automation prevailed.