Guid generating bug in C#
===

I was working with Guid.NewGuid() in C# , .NET 4.0 . And I've found interesting fact: every Guid I've generated has '4' at 13 position.

    da1471ac-11f7- 4 fb7-a7fa-927fffe8a97c
    c90058aa-5d7f- 4 bb5-b3a9-c1db197cf3b1
    fa68ec75-8cd2- 4 c24-92f8-41dbbdd428a0
    d4efd455-e892- 4 3ef-b7bf-9462c5dc4de4
    e0a001a0-8969- 4 092-b7a2-e410ed2b351a
    30ae98b9-48ae- 4 25d-b6e7-e091502d6ce2
    6a95de82-67ff- 4 4c9-9f7b-e37a80462cf7
    66768e46-6d60- 4 2b4-b473-2f6f8bc1559a

I've tried it on several machines and have the same result.
Can anybody try it too or explain it?

Simple code for checking:

    static void Main(string[] args)
            {
                bool ok = false;
                for (int i = 0; i < 10000; i++)
                {
                    var guid = Guid.NewGuid();
                    if (guid.ToString()[14] != '4')
                        ok = true;
                    Console.WriteLine(guid);
                }
                Console.WriteLine(ok ? "No bug!" : "4bug founded!");
            }