SD Billing Document Number Ranges

Hello SD Guru's

My clients requirement is like he has four billing type for export he want it to have separate invoice series for each billing type but with same sales doc type .He want to select billing type manually in VF01 .(At the time of SO creation, we might not know the export type. So, we want to select the Billing Type, in VF01.) And he want each one billing type should have different number ranges .Can it be possible through Validation if yes please guide as i have not done validation in SD so please Revert.

Thank You In Advance.