If a salaried employee is out on FMLA and comes back for 4 hours a day - does payroll charge them with 4 hours sick/vacation time - or because they are salaried do they get full pay. Thank you!

Even exempt employees ("salaried" is merely a pay method) may be docked for partial day absences related to FMLA.
http://www.dol.gov/dol/allcfr/ESA/Title_29/Part_541/29CFR541.602.htm

You do not have to substitute with vacation or sick time, but you may.