Could a 20 mg dose of prednisone still be in your system after 4 days?

No: Prenisone is converted to prednisolone in about 1 hour. Half of what remains is excreted in the urine in 2-4 hours (half-life) unless. So after 4 hours, you have 10mg in your system. After 8 hours, you have 5mg, after 12 hours, 2.5... You can see that the amount in your system after 4 days would be very very small. For therapeutic purposes, it would be gone. For a drug test? I don't know.