Bumping this topic, because, after much procrastination, I finally decided to upgrade from MIAB 0.30/Unbuntu 14.04, which requires a new install of Ubuntu 18.04 and encountered the same issue: nsd4 would not start because IPv6 was disabled
In my case, my VPS provider’s (Contabo) OS Re-install feature actually disables IPv6 by default, without letting me know.
I was very lucky to come across this old, but precious post and commented out the line below in my /etc/sysctl.conf file which fixed the issue:
#net.ipv6.conf.all.disable_ipv6 = 1
Obviously, as with the original poster, I would really like to understand why disabling IPv6 prevents nsd4 from starting.