You can't use Phenol red as an indicator solution and still use 4 dKH water. That KH was chosen to make the solution be at it mid-point on the indicator color chart at 30 ppm of CO2. Phenol red's mid-point is about 7.3 pH, which would require a 20 dKH solution in the bulb to reach the mid-point at 30 ppm of CO2.

And, it is important that the drop checker be entirely submerged in the tank water, to avoid any distillation between the water in the drop checker and the water in the tank.

I think you have the ppm of NaHCO3 wrong also, but I could be mistaken. I think 1 dKH = about 18 ppm of CO3. I admit that I am no chemist, and I get the relationship between ppm of sodium bicarbonate and dKH confused.