Assigning random letters to several jbuttons

I need to have 4 random letters assigned to 4 random jbuttons out of 16. What I currently have is putting the same random letter on all 16 buttons. Thanks for the help!

package code;

import java.awt.GridLayout; import java.util.Random; import javax.swing.JButton; import javax.swing.JFrame; import javax.swing.JPanel;

public class GameG implements Runnable {

    public static JButton jba;  public static JPanel jp;

@Override public void run(){

    JFrame jf = new JFrame("Keybricks");
    Random rnd = new Random();

    jp = new JPanel();

    jp.setLayout(new GridLayout(4, 1));

    jf.setSize(300, 600);
    jf.setVisible(true);
    jf.setResizable(false);
    jf.add(jp);
    jf.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
       char[] letter = new char [1];
     for (int i = 0; i < letter.length; i++) {    char c = (char) (rnd.nextInt(26) + (65));       letter[i] = c;   } 

    String text = new String(letter);
     JButton[] buttons = new JButton[16];
     for (int i=0; i < buttons.length; i++) {
          JButton jba = new JButton(text);
      jba.addKeyListener(new KeyL());
      jp.add(jba); 

        }   }  }