I'm in doubts the probability of rolling "10000" is 0.0001 with current algorithm, it would be interesting if someone would do an independent test (run an implementation of current algorithm 1,000,000 times, look at the amount of 10000's).

upd Ok, I looked at algorithm closer, and if everything is properly randomized, we can calculate the probability theoretically:

"3. The first 8 characters of the hex string are taken and converted to a decimal.
4. This decimal is then divided by 429496.7295 and rounded up to the nearest whole number.
5. This whole number is used as your roll, with the maximum possible value being 10,000."

We have hex strings from "00000000" to "ffffffff" (0 to 4294967295), 4294967296 in total. "ffffffff" converts to 4294967295/429496.7295 = 10000.

The biggest number which will be rounded to 10000 is 9999.5, back converting it (*429496.7295), we'll get 4294752546.63525.

So the numbers from the interval [4294752547, 4294967295] will be converted to "10000", the quantity of them = 4294967295-4294752547+1 = 214749.

The probability of getting "10000" = 214749/4294967296 = 0.00005000014789402484893798828125 (~ 1/20000)

P.S. After 40,000 rolls one should get 86% probability of rolling "10000" at least once (proof: https://www.wolframalpha.com/input/?i=1-%281-0.00005000014789402484893798828125%29^40000)