First off, the API kits are not the best for Nitrates. You may have higher levels than what it indicates. Second, are you running a skimmer? If so, that is reducing your nitrates some. Alone, it wont get rid of all but could account for the lower reading than you would expect to see.
, if you dose to 4 ppm
ammonia and in 24 hours you have no ammonia and/or nitrite, you are ready to do some serious water changes and add livestock. Just add slow. One or two fish then give it a couple weeks before adding any more.