A 0.4 pH swing between day and night is not uncommon at all for planted tanks. Ponds and rivers can experience much greater drops and much larger CO2 levels than we see in a given day. I have the same 0.4 pH swing daily too, and have for several years. It does bother some people, if it bothers you then there are two easy solutions.
1) Run an airstone to come on when the lights go out. (controlled by a timer).
2) Add a solenoid to your CO2 regulator and use a timer to shut off your CO2 when your lights go out, many people (including Tom Barr) don't run they CO2 24/7.
And lastily, but a bit more expensive and problematic is to use a pH controller. While the controller can maintain a constant pH, it does need regular calibration and is subject to failure which can result in CO2 being dump at lethal amounts.