ntopng support HTTP binding over both IPv4 and IPv6, so you can connect to your ntopng web GUI using both IP versions.
At the moment you can specify the listening ports for HTTP and HTTPS using --http-port=1234 and --https-port=5678
but you cannot specify if you want to listen only on IPv4 or only on IPv6: ntopng will listen on both IP versions
is available (or only on the available IP version).

If you see that ntopng does not listen on IPv4 please check the following setup

Linux
sysctl net.ipv6.bindv6only=0 

FreeBSD
sysctl net.inet6.ip6.v6only=0

See http://man7.org/linux/man-pages/man7/ipv6.7.html for more information.