List<string> splitList = input.Split(';').ToList();
splitList.RemoveAt(3);
//Re-create the string
string outputString = string.Join(";", splitList);
//output is: "one;two;three;five;six"
List<string> splitList = input.Split(';').ToList();
splitList.RemoveAt(3);
//Re-create the string
string outputString = string.Join(";", splitList);
//output is: "one;two;three;five;six"