code: @Override
  	public IPv4Address toIPv4() {
  		IPAddressConverter conv = DEFAULT_ADDRESS_CONVERTER;
  		if(conv != null) {
  			return conv.toIPv4(this);
  		}
  		return null;
  	}
docs: If this address is IPv4 convertible, returns that address.
  Otherwise, returns null.
  <p>
  This uses {@link #isIPv4Convertible()} to determine convertibility, and that uses an instance of {@link IPAddressConverter.DefaultAddressConverter} which uses IPv4-mapped address mappings from rfc 4038.
  <p>
  Override this method and {@link IPv6Address#isIPv4Convertible()} if you wish to map IPv6 to IPv4 according to the mappings defined by
  in {@link IPv6Address#isIPv4Compatible()}, {@link IPv6Address#isIPv4Mapped()}, {@link IPv6Address#is6To4()} or by some other mapping.
  <p>
  For the reverse mapping, see {@link IPv4Address#toIPv6()}