I moved this to a new topic because it’s interesting information.
Right now, we don’t adjust the DNS entry for <app>.fly.dev when you add or remove IPs, it’s always pointed at the first two IPs we provision. That’s something we plan to change. If you remove your ipv4, <app>.fly.dev will only work for ipv6 lookups at that point.
When we ship shared IPs, you’ll be able to add a shared IP to your app just like a normal ipv4.
Also the dedicated v6 IPs are free.