Question
When we hit 10,000 jobs, they started sorting as if it was 1 again. That is tricky because when you are sorting by job number, it appears as if the most recent job is 9,999. The jobs in the 10,000 are sorted as much older (even before the jobs in the 2000 numbers)
Answer
The auto-number fields are sorted alphabetically, not numerically - so the sorting is based on the first character in your job number.
When you only had four digit numbers, every was sorting fine, but once you added the fifth digit, things changed. The sorting looks something like:
9999 9998 ... 1001 1000 10000 0999 0998
To prevent this on other auto-number fields in the future, you can set the minimum number of digits to be 5 or more, but you can't change the existing job numbers.