NBN Co chief network engineering officer Peter Ryan described this as "a significant rollout milestone".
A media release from the company said by the end of December 2014, 778,824 premises had been made ready for service. In the next two years, an additional 3.22 million premises had been readied for service.
It said four technologies had been used after 2014: FttB, FttN, HFC and Sky Muster satellite broadband.
Ryan said: "Reaching four million premises is a great achievement and is a testament to the hard work of our people and our delivery partners.
"We now have the flexibility and the right technologies in place to design and build the network at the speed and scale needed to reach our end goal by 2020.
"With these four million premises our halfway built milestone comes into focus – we are expecting to reach this around the middle of the year."