using System;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		String original = "this is your sensitive digit 12345 , if you have any question please call 123456789";
		String pattern = @"(?<!\d)(?!123456789(?!\d))\d{4,}";
		String result =  Regex.Replace(original, pattern, "****");	
		Console.WriteLine(result);
	}
}