what was the number of patients that were given a phenytoin, percent free test since 4 years ago?
select count( distinct admissions.subject_id ) from admissions where admissions.hadm_id in ( select labevents.hadm_id from labevents where labevents.itemid in ( select d_labitems.itemid from d_labitems where d_labitems.label = 'phenytoin, percent free' ) and datetime(labevents.charttime) >= datetime(current_time,'-4 year') )