A Swiss VM hosting provider has a technical blog post about how to kill IPv4 completely on FreeBSD [ungleich.ch]. That is to say, turning it completely off, not just preferring IPv6. They then solicit concrete soutions describing, along with a proof of concept, how turn IPv4 completely off in other operating systems and allowing them to communicate with IPv6 only.
Earlier on SN:
Vint Cerf's Dream Do-Over: 2 Ways He'd Make the Internet Different [soylentnews.org] (2016)
You have IPv6. Turn it on. [soylentnews.org] (2016)
We've Killed IPv4! [soylentnews.org] (2014)