(Java) Reading the Serial Port with the Jssc library

I have a java app that le in the serial port with 7 data from a rotation sensor coming from the arduino, via jssc. The code works, but the data is not formatted. I read 4 by 4 bytes, but sometimes it comes with numbers like 0080, 8000, etc. I need to leave the output formatted, how can I do this? Follow the code.

package tanio;

import java.io.UnsupportedEncodingException;

import javax.swing.SwingWorker;

import jssc.SerialPort;
import jssc.SerialPortException;
import jssc.SerialPortList;

public class obterrpm extends SwingWorker{


        protected Object doInBackground() throws Exception {



              String[] portNames = SerialPortList.getPortNames();
                for (String portName : portNames) {
                 //   System.out.println(portName);
                }
                    SerialPort serialPort = new SerialPort("COM7");

            try {
                serialPort.openPort();
                serialPort.setParams(9600, 8, 1, 0);
                //System.out.println("successfully writen to port: " + serialPort.writeBytes(new byte[]{0x04}));
                while(true){
                delay.delay(100);
                byte[] buffer = serialPort.readBytes(4);//Read 10 bytes from serial port

              String convertido = new String(buffer);

               //System.out.println(convertido);
              chamarTelaPrincipal.rpm.setText(String.format("%s",convertido));

               // Runnable enviarResposta2 = new SocketRespostaRPM(convertido);
                //  new Thread(enviarResposta2).start();



                }
                //System.out.println("Port closed: " + serialPort.closePort());
            } catch (SerialPortException ex) {
                System.out.println(ex);
        }


            return null;
        }

    }