Add mod 10000 will give bias because values above 10000 will contribute to interval 0...n-10000. Also remember bit flipping. With block cipher you can encrypt value between 0 and 10000 to a value between 0 and 10000 and it will be PRP. With stream cipher you can't do that. Bock cipher allows to encrypt points of quite weird domain to the point of that domain. Ex. you can encrypt point on curve to another point on this curve and it will be PRP. And there is a theorem that proves that if PRP is used with cycle- walking mode of operation, the resulting transformation is also PRP.
-Valery.