How to add br in between the string in List string

.Net 2.0 - .Net 3.5

        List<string> arr = new List<string>();
        string s = "";
        arr.Add("1");
        arr.Add("2");
        arr.Add("3");
        arr.Add("4");
        for (int i = 0; i < arr.Count; i++)
        {
            s += arr[i] + "<br />";
        }        

.Net 4.0

string ss = string.Join("<br />", arr);