If the drop checker has 4 dKH distilled or DI water in it, you have from about 25 pm to about 40 ppm of CO2 in the water, so that should be enough. But, adding more, and having the pH drop below 6.5 is not a problem due to the pH, but the added CO2 could cause the fish some distress. GH is a measure of the ppm of calcium and magnesium in the water, and the test for that is usually accurate when expressed in degrees of hardness.

When I allow my plants to grow and grow and grow until the tank gets really full of big healthy plants I get BBA, because the water can no longer circulate through all of that greenery, even though I use a powerhead too, just for water circulation. I have seen this happen several times now. It is disappointing because I really enjoy the look of a tank filled with healthy plants. That is why I wonder if you are seeing something similar.