Combination of 4 alphabet character and 4 digits

string s = "12345678";
string toAlpha = s.Substring(0, 4);
string toDigit = s.Substring(4);
StringBuilder sb = new StringBuilder(8);
foreach (char c in toAlpha)
    {
    sb.Append((char)('A' + (c - '0')));
    }
sb.Append(toDigit);
string output = sb.ToString();

private string Encode(string numeric)
    {
    string alphaPart = numeric.Substring(0, 4);
    string numPart = numeric.Substring(4);
    uint u = uint.Parse(alphaPart);
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < 4; i++)
        {
        sb.Insert(0, (char)('A' + (u % 26)));
        u /= 26;
        }
    sb.Append(numPart);
    return sb.ToString();
    }
private string Decode(string mixed)
    {
    string numPart = mixed.Substring(4);
    string alphaPart = mixed.Substring(0, 4);
    uint u = 0;
    foreach (char c in alphaPart)
        {
        u *= 26;
        u += (uint)(c - 'A');
        }
    return u.ToString("0000") + numPart;
    }