public static void enableLog4jMonitor(Map<String, String> configsMap){

    String log4jpath = AmbariServer.class.getResource("/"+Configuration.AMBARI_LOG_FILE).toString();
    String monitorDelay = configsMap.get(Configuration.LOG4JMONITOR_DELAY.getKey());
    long monitorDelayLong = Configuration.LOG4JMONITOR_DELAY.getDefaultValue();

    try{
      log4jpath = log4jpath.replace("file:", "");
      if(StringUtils.isNotBlank(monitorDelay)) {
        monitorDelayLong = Long.parseLong(monitorDelay);
      }
      PropertyConfigurator.configureAndWatch(log4jpath,  monitorDelayLong);
    }catch(Exception e){
      LOG.error("Exception in setting log4j monitor delay of {} for {}", monitorDelay, log4jpath, e);
    }
  }
To change log level without restart.
ambari-server/src/main/java/org/apache/ambari/server/controller/AmbariServer.java