Sony has revealed in a PlayStation blog post that 4.2 million PlayStation VR headsets have been sold worldwide up to March 3. The PlayStation VR device was released in October 2016 and had passed one million sales by June 2017. A new Iron Man VR game has also been announced, which should help Sony shift a few more PSVR units.
https://www.notebookcheck.net/Sony-announces-4-2-million-PlayStation-VR-headset-sales-Iron-Man-VR-should-help-sell-some-more.415095.0.html