char[] chars = new char[]
{
    (char)0xd83d, (char)0xde04,
};
byte[] bytes = Encoding.UTF32.GetBytes(chars);
Console.WriteLine("bytes.Length = " + bytes.Length);

foreach (var b in bytes)
{
    Console.WriteLine(b.ToString("x2"));
}

bytes.Length = 4
04
f6
01
00

bytes.Length = 8
04
f6
01
00
00
00
00
00