Use jTextField4.setEditable(false);

Here is an example:

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
class TextFieldEditable 
{
    public static void main(String[] args) 
    {
        JTextField jTextField1=new JTextField(20);
        final JTextField jTextField2=new JTextField(20);
        final JTextField jTextField3=new JTextField(20);
        final JTextField jTextField4=new JTextField(20);
        JButton b=new JButton("Calculate");
        JPanel p=new JPanel(new GridLayout(5,1));

        b.addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent evt){
    if(jTextField2.getText().equals("")||jTextField3.getText().equals(""))
    {
        JOptionPane.showMessageDialog(null,"Please enter the fields to Calculate","Error",0);
    }
    else
    {

     String total=jTextField2.getText();
        String paid=jTextField3.getText();
     int i = Integer.parseInt(total);  
     int j = Integer.parseInt(paid);
     int k=i-j;
      String balance = new Integer(k).toString();
      jTextField4.setText(balance);
    }
            }
        });
        jTextField4.setEditable(false);
        p.add(jTextField1);
        p.add(jTextField2);
        p.add(jTextField3);
        p.add(jTextField4);
        p.add(b);

        JFrame f=new JFrame();
        f.add(p);
        f.setVisible(true);
        f.pack();
    }
}