Best way to convert seconds into (Hour:Minutes:Seconds:Milliseconds) time?

Just use the TimeSpan class.

TimeSpan t = TimeSpan.FromSeconds(secs);

string answer = string.Format("{0:D2}h:{1:D2}m:{2:D2}s:{3:D3}ms", 
 t.Hours, 
 t.Minutes, 
 t.Seconds, 
 t.Milliseconds);