No field for severity , for logs coming from log4j

Elastic Stack Logstash
1

There is no field for Severity/Priority in log4j logs.
Logstash version 2.2.x

log4j config:

    log4j.rootLogger=stdout
    log4j.appender.stdout=org.apache.log4j.net.SocketAppender
    log4j.appender.stdout.port=5050
    log4j.appender.stdout.remoteHost=x.x.x.x

logstash config :

input {
   log4j{
      port =>5050
      type => 'log4j'
      codec => "json"
        }
}

output {  
 elasticsearch {
       hosts => ['172.30.0.148:9200']
          }
 }

And a log looks like:

@timestamp	  	July 1st 2016, 11:30:18.775
t@version	  	1
t_id	  	AVWlCybQRaRyYuJtS1Ke
t_index	  	logstash-2016.07.01
#_score	  	
t_type	  	log4j
thost	  	x.x.x.x
?message	  	 ZmdcCopyLookupRequiredZndcLookupRequiredJ	timeStampLcategoryNametLjava/lang/String;LlocationInfot#Lorg/apache/log4j/spi/LocationInfo;LamdcCopytLjava/util/Hashtable;Lndcq~LrenderedMessageq~L
?port	  	  60165
?tags	  	  _jsonparsefailure
?type	  	  log4j

I used a filter from below git page

But there were configuration errors and logstash stops running while using this filter.

How can I add severity field here?