Prednisone is known to raise glucose levels, the dosage amount also plays a large role. I disagree with the ER part because you didn't tell us if you are taking insulin, if this was one time reading or multiple readings. A one time reading does not warrant a trip or call to ER. It does, however, in your best interest, warrant a call to your doctor to see if your medication requires adjustment. Good luck.
I would call the ER or go down to them 400 is way to high