UnrealIRCd makes use of the "dual IP stack" of your system, meaning it can listen on both IPv4 and IPv6 addresses. UnrealIRCd 4 is always built with IPv6 support. To actually use IPv6 you'll need an IPv6 address on your machine.
UnrealIRCd has IPv6 support since 2003 but in 3.2.x it was an optional feature and there was no IPv6 support on Windows. With UnrealIRCd 4 this changed: Windows support was added and IPv6 support is always compiled in.