Can anyone help me with the ammonia problem in my tank?

i have been cycling my 20 gallon tank for nearly four weeks. all the nitrate and nitrite readings are at zero and my ph level is around 6.8. the only thing that is an issue the ammonia which about 4.0ppm, very bad for any fish if i were to add them at the moment. i have tried everything to lower it, including adding bacteria supplements, ammo-lock, making frequent water changes (usually, the ammonia levels will shoot up a day or so after changing it), nothing seems to help it. so i'm wondering should i just leave it alone and hope that things will work out or is there something i can do to lower it. 4 weeks seems like a long time for a tank to be cycling, when i have had other tank cycle in 2 weeks. thanks for your help!