All I know is my 4Dkh solution reads in the vacinity of 7.6 pH with ambiant CO2 so it must be something else. If it's PO4 then no. The KH in a drop checker must get its hardness from carbonate or the KH/pH/CO2 chart relationships are invalid. Might as well use tank water.
Add 6g oven dried (325 deg. 1 hour) baking soda to 1000ml distilled water. = 200 Dkh water
Measure 490 ml distilled water and add 10 ml of the 200 Dkh water. = 4 Dkh water
Measure 390 ml distilled water and add 10 ml of the 200 Dkh water. = 5 Dkh water
Many of may tanks have KH of around 4-5 degrees, and a pH ranging though the upper 6s and low 7s.
I do not think water with KH of 4 degrees could have a pH of 4. Not saying it is impossible, but I do not see that.
Even my tanks that show no KH have a pH of about 6.0