Kurz und bündig:
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Code:
            String testString = "1 / 2 / 3 / 4 /";
            Boolean replace = false;
            for (int i = 0; i < testString.Length; i++)
            {
                if (testString[i] == '/')
                {
                    if (replace)
                    {
                        testString = testString.Remove(i,1).Insert(i, "\r\n");
                    }
                    replace = !replace;
                }
            }
            MessageBox.Show(testString);