Yup, what
@Grizzly said... the issue is that as you travel with your mobile from tower to tower, it depends what part of the network is IPv6 and what is IPv4, basically it works like 4G to 3G automatically. Where you don't have high speed connection, it reverts to the lesser connection.
Until countries get their entire mobile networks switched over to IPv6, this will continue being an issue. Saying that, they will, for a long time anyway, still have IPv4 as a fallback, as you will have lots of older phones still connecting that don't necessarily have IPv6 capabilities. The majority of phone users though, are already using it where your ISP has it, automatically. You may also have country areas where they can't justify the cost... so they will remain on the IPv4.
ISP's, hosting companies, basically anyone that issues IP's to people, have all reigned them in severely, literally taken them away without notice if they aren't used IAW the policies for them. For example, this site has its own IP due to running SSL, as does MyPTSD. I then have the servers NS1 & NS2 IP's, so that is four. I had six, they took one away without telling me, they will take the other also, no doubt... and I will only get them back if I specifically plan to put a site on SSL and thus need the IP.