A secret larger than 10,000 is not useful -- since the multiplication is mod 10,000, the first digit of the secret could be ignored anyway (54321 = 5 * 10000 +

A secret larger than 10,000 is not useful -- since the multiplication is mod 10,000, the first digit of the secret could be ignored anyway (54321 = 5 * 10000 + 4321 === 4321 mod 10000).