Could allergy testing be done 4 days after being given a 10mg injection of decadron (dexamethasone)?

Not accurately.: Decadron (dexamethasone) is a pawerful anti-inflammatory steroid, which has a half-life of about 36 hours, which means on day 4, you have the equivalent of 2 mg of Decadron (dexamethasone) in your body, which is enough to decrease skin testing. Serum testing is also innaccurate at this point, will need to wait another 2 weeks for skin testing, up to 6 weeks for blood testing.