Serial Numbers for different categories

Dear All,

I am having different categories and I want to give Serial numbers for each category. For example there are categories AA, BB and CC. For each category I would like to give serial numbers like for AA starting from 1000 and for BB-2000 and CC-3000. (please refer the attached file for better understanding of the situation)

I want to do it using formula.

Any help appreciated.

Thanks,
aganesan99