Convert mapped IPv6 to IPv4

Elastic Stack Logstash
2

Combining the referenced ruby filter code with Ruby conversion code, I'm trying out the following piece of code in my filter:

if(![srcIPv4] and [srcIPv6]) {	
			mutate { add_field => { "srcIPv4" => ""}}
			ruby {
    			init => 'require "ipaddr"'
  			    code => '
  			        	ipv6 = IPAddr.new(event.get("srcIPv6"))
  				        event.set("srcIPv4",ipv6.native)'
			}
		}

I've used a workaround using [geoip][ip] but I think I'm paying a high computational price for IPv6-to-IPv4 conversion.

The error that I'm getting in logstash's logs, because of this, is:

[2018-12-01T17:34:13,309][ERROR][logstash.filters.ruby    ] Ruby exception occurred: Missing Converter handling for full class name=org.jruby.RubyObjectVar2, simple name=RubyObjectVar2

And a link with the Ruby code working in the tester.

Any help appreciated, thanks!