It could well read that same 4 ppm for a couple of weeks. That does not mean that there is anything wrong. You are trying to multiply the bacteria that happened to be in the water that you used to fill the tank. If the final amount needed for your filter is let's say a half a million, and the bacteria double each day, the day before you have enough there will only be half enough, the day before that only 1/4 enough, etc. On day 1 you might only have a very small number but doubling every day does eventually go fast enough to get where you are going. It goes 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, etc. for each original bacterium. As you can see, by day 11 each original bacterium has become about 2000. After that things will seem to speed up.