at KH of 5.5 (and pressurized co2) I get about a .05 change in pH at night, but nothing more. Therefore it stays between 6.70 and 6.75 at all times.
Do you have anything in your tank that adds to KH over time, like Limestone, Crushed Coral, or anything else? I had to take out about 50% of my CC in my Fluval due to the variations in KH over days at a time.
Edit>> should have actually asked whether or not you have re-tested KH each time you got those measurements, or if you assume it stays constant at 4.
450g Planted Discus Tank - Work in Process!