Have you added any ammonia during the 4 weeks the tank has been running? While the ammonia and nitrite levels look perfect, this is because you haven't added anything which might make them not-perfect. The Denitrol may or may not contain the right species of live bacteria, but the only way to be 100% sure is to test it. The danger is that with no food in the form of added ammonia, the bacteria in the bottle won't have multiplied to make enough bacteria yet.
Unless the tank is heavily planted, I suggest you get some ammonia or Dr Tim's ammonium chloride and add enough to get a reading of 3 ppm half an hour after adding it - this is to allow it to mix in thoroughly. If you use a bottle of ammonia, there is a calculator on here to work out how much to use (the last item)
https://www.fishforums.net/aquarium-calculator.htm Test again after 24 hours. If both ammonia and nitrite are zero, the tank is cycled. If one or both are not zero, you need to continue with a fishless cycle
https://www.fishforums.net/threads/cycling-your-new-fresh-water-tank-read-this-first.421488/
Make sure that any ammonia is pure - just ammonia or ammonium hydroxide in the ingredients, no perfume, detergent etc.
However,
if the tank is heavily planted, and I mean more than just the odd couple of slow growing plants, then as long as the plants are visibly growing, you can add the first batch of fish. To be safe, check ammonia and nitrite every day till you are sure they are staying at zero. After week of zeros, you can get the next batch of fish, and again check those levels every day
