Typically a nurse can only pass on the diagnosis from the doctor. When the nurse says "good," I expect that is what the doctor is passing on through the nurse to you.
In addition, as others have said, there are two other factors in your case (1) you're only 4 weeks out of surgery and (2) we don't know the sensitivity of the tests that your doctor uses. Many doctors wait 6-8 weeks for the first PSA and also say, "don't worry about the first one," since your PSA level in the blood may not have stabilized.
I truly believe you're just fine right now. Also you have all the license in the world to call back and ask the nurse to pass questions on to the doctor if you are concerned. You are in the driver's seat. Get the answers you want from your health care professionals.
All the best,