what's the minimum total cost of the hospital that involves a drug called sodium acetate until 4 years ago?
select min(t1.c1) from ( select sum(cost.cost) as c1 from cost where cost.hadm_id in ( select prescriptions.hadm_id from prescriptions where prescriptions.drug = 'sodium acetate' ) and datetime(cost.chargetime) <= datetime(current_time,'-4 year') group by cost.hadm_id ) as t1