<window xmlns:w="http://www.zkoss.org/2005/zk/client" width="740px">

  <textbox id="message" multiline="true" width="540px" height="150px">

    <attribute w:name="onKeyUp"><![CDATA[

      var val = this.getInputNode().value,

          cleanList = [],

          tempList = val.split('\n');

      for (var i=0; i<tempList.length;i++) {

        var cleanS = tempList[i].substring(0,4);

        cleanList.push(cleanS);

      }

      if (cleanList.length > 5) {

        cleanList.pop(); //remove last line

      }

      this.setValue(cleanList.join('\n'));

      zk.log(cleanList);

    ]]></attribute>

  </textbox>

</window>

</zk>