Sure, Nyquil will boost your blood alcohol leve. since it contains alcohol itself. You should have told your work that you had taken Nyquil; they might have been more understanding.
The steroid shot didn't help, either. Steroids decrease the rate of metabolism of alcohol, making it slower to be metabolized by the body.
A BAC of .04 is only half that (.08) required to be classified as legally impaired. If you had told your work the circunstances, you probably wouldn't have lost your job. Can you appeal the decision, or ask for a retest?