For a start ammonia is not measured in drops unless you have siomething like Dr. tim's ammonium chloride which is packaged in a bottle that dispenses in drops. There is an ammonia calc on this site that tells one how many ml of ammonia to add to a tank of X size in order to produce Y ppm. In your case using 10% ammonia to produce 4 ppm in one gallon takes .15 ml of ammonia. There are approximately 20 drops in 1 ml. So you should have about 6 and 2/3 gallons of water in your tank? I bet that is not the case?
I distrust the nitrate kits. Assuming 0 nitrate in your tap then to make 40 ppm of nitrate you need to start with about 11 ppm of ammonia. This should take multiple additions during a cycle and 0 water changes. In the absence of other influences 1 ppm of ammonia becomes 2.56 ppm of nitrite and then 3.46 ppm of nitrate.
Next, the biggest issue I have is with your reporting a steady 4 ppm level. This simply is not possible for a whole host of reasons:
1. The plant would use some of the ammonia and if it is too much, the plant will suffer or even die. Too much ammonia is as bad for plants as it is for fish.
2. Ammonia is gas that will evaporate over time. This is why using store brands can have the issue of varying strength. $ week in a tank with surface aggitation should lower ammonia some.
3. Even if you are wrong and have overdose ammonia, there are still bacteria that will process at higher levels. they are not the ones wanted long term in a tank, rather they are like the ones that are found in waste water treatment. Some of them would have begun to work in a tank with too much ammonia.
4. Assuming a test kit is accurate (this includes proper use of it), if one has X ammonia in a tank and changes 25% of the water adding ammonia free water back, then the ammonia reading should drop by 25% to .75X. In your case a 25% wc should drop ammonia to 3 ppm and a 2nd 25% would further lower that to 2.25 ppm.
What all of the above tells me is that the results your are reporting simply cannot be real/accurate. The are not possible. Trying to find out why you are getting these results is not so easy remotely. Over 4 weeks there are many things you may have done including some mistakes. You could be testing wrong or your kit could be faulty. The way I like to show that these reading simply cannot happen is to challenge folks to try to replicate them. Could you figure out a way to produce the results you appear to be getting? Could you make a tank test for 4 ppm of ammonia 24/7 for 4 weeks?
I reread you other thread too. Nowhere in that one or this one do you mention a number of relevant things, most importantly what size tank you have?? Here is other info to provide that should help:
1. Tap parameters: pH, KH, Nitrate, Ammonia
2. Tank parameters: pH, KH, Temp.
3. Brand of decchlor.
4. Ammonia dosing and water change history.
5. Anything else added to the tank such as ferts., bacteria, etc.