Force Inbound Connections to IPV6 only
onlynet only affects outbound connections.
If you only want inbound IPv4 connections (why?), you'll need to either:

Patch Bitcoin Core to bind to IPv6 addresses in IPV6_V6ONLY mode (see CConnman::BindListenPort in src/net.cpp)
Use some firewall rule on your system to prevent incoming IPv4 connections to it
Make sure you only have a publicly-reachable IPv6 address