The sum and product of a positive number between 1000 and 9999.

I wrote a program that asks the user for a number and displays the original number, the sum of the digits and the product of the digits. I need to add an ERROR message if the number entered is not positive or between 1000 and 9999. I also need it to display "Zero" if the product of the four digits is 0. I am just trying to figure out the easiest way of accomplishing this task. This is what I have so far:

import javax.swing.JOptionPane;

public class Program2
{
public static void main(String[] args)
{

String userInput;
int posFourDigits;
String output, output1, output2;

userInput = JOptionPane
.showInputDialog("Enter a positive four digit number between 1000 and 9999: ");
posFourDigits = Integer.parseInt(userInput);

output = "The original number is: " + (posFourDigits) + "\n";
output1 = "The sum of the four digits is: "
+ ((posFourDigits % 10) + (posFourDigits / 10 % 10)
+ (posFourDigits / 100 % 10) + (posFourDigits / 1000))
+ "\n";

output2 = "The product of the four digits is: "
+ ((posFourDigits % 10) * (posFourDigits / 10 % 10)
* (posFourDigits / 100 % 10) * (posFourDigits / 1000))
+ "\n";
JOptionPane.showMessageDialog(null, output + output1 + output2,
"Program 2 (Artemus Watson)",

JOptionPane.PLAIN_MESSAGE);

System.exit(0);
}
}