Ok, you mean ammonia hydroxide. Not ammonium hydroxide.
More questions for you,
1. Have you been kept adding ammonia? Or did you only add it once in the last one month?
2. Was ammonia always at 4ppm during the entire month?
3. How long exactly have you been doing this fishless cycling?
You said 3~4 weeks. If it is really just 3 weeks, it is perfectly normal for "nothing happens". I have been there. It took 3~4 weeks to see the first sign of nitrite.
4. When did the nitrite appear? When did it disappear?
Since you are using API master kit, I would like to make sure you have used it correctly.
The nitrate test of this kit require special procedure. You must shake the test bottles for up to 1 minute really hard before adding them into the testing tube. After you mixed them up, once again you must shake the tube really hard for up to a minute before letting it to settle down. Wait a few minutes to see the result.
Fail to do the two "shaking" steps above will result in false 0ppm nitrate reading. One time I have forgotten to shake the bottles first, the result was 0ppm nitrate while it shouldn't be. After I retested it with correct procedure, I got a reading.
Not ammonium hydroxide. 
