fill randomly 4 time a grid in java with letter

Good evening, I have to fill in a grid that I created by adding 4 random letters in it. I don't know how to get started, I have an array [i=5][j=4] and I have to add to a random i and j a G and that 4 times. I am aware of having to use Math.random() as well as a while(x<4) but I don't know how to go about it.enter image description here

package tp1;

import java.util.*;
import java.lang.Math;

public class Joueur {

  //attributs

  String pseudo;
  private char[][] grille;
  private int nbreManches;
  private int nbreObstacles;

  //constructeurs

  public Joueur(String valeur) {

    nbreManches = 0;
    nbreObstacles = 0;
    grille = new char[5][4];
    pseudo = valeur;

  }

  public Joueur() {

    Scanner sc = new Scanner(System.in);
    System.out.println("Quel est votre pseudo ?");
    pseudo = sc.nextLine();
    nbreManches = 0;
    nbreObstacles = 0;
    grille = new char[5][4];

  }

  public void afficher() {

    grille = new char[5][4];
    System.out.println("************************************");
    System.out.println("Pseudo: " + pseudo);
    System.out.println("Nombre de manches: " + nbreManches);
    System.out.println("Nombres d'obstacles: " + nbreObstacles);
    System.out.println();


    for (int i = 0; i < 5; i++) {
      for (int j = 0; j < 5; j++) {
        System.out.print("|");
      }
      System.out.println("|");
    }
    System.out.println();


  }


}