Added functionality on button press for the fourth button

src/main/java/game/StartTicTacToeGame.java

@@ -203,5 +203,23 @@ public class StartTicTacToeGame implements ActionListener{
 				player2button3Pressed = true;
 			}
 		}
+		else if(e.getSource() == button4) {
+			if(player1Turn) {
+				button4.setForeground(Color.black);
+				button4.setText("X");
+				button4.setFont(new Font("arial", Font.PLAIN, 30));
+				player2Turn();
+				button4.setEnabled(false);
+				player1button4Pressed = true;
+			}
+			else {
+				button4.setForeground(Color.black);
+				button4.setText("O");
+				button4.setFont(new Font("arial", Font.PLAIN, 30));
+				player1Turn();
+				button4.setEnabled(false);
+				player2button4Pressed = true;
+			}
+		}
 	}	
 }