Just for the record and maybe you like to know.. But it actualy is a bit nitpicking and it probably wont influence the reading that much or not at all. Since this pH measuring also just is an approximate vallue and the same with thermometers, if you take 3 different meters you likely will have 3 different readings and impossible to know which one is most accurate. So it al aint realy of such importancy..
But when it comes to calibration the meter should be calibrated closest to the target sample one wants to measure. That's why there are several calibration buffer solution available.4, 7, 9.6, 10 etc. Now the manufacterer makes an assumption about what you want to use it for, since it is a ph meter the assumption is you want to measure acidity.
Most are therefor used in the horticultur where fert sulotions generaly are made acidic depending on what you want to grow down to pH 5 can be recomended. According to this assumption they recomend buffer solution 4 and 7..
Now these meter have a degree deviation this deviation is lineaire the further away we are from the sample the more this devation can be. So to be most accurate should be calibrated near the expected sample reading. Do we want to make the sample pH 5, we need to calibrate first with pH7 as zero point calibration and secondly with pH 4 to be closest to the sample. Do we want a sample from pH 8 then we should calibrate secondly with a buffer of pH 9.6 or pH10 to get most accurate reading.
In the aquarium hobby almost all want a nutral as possible sample, even if we use co2 we stay much closer to pH 7 than we would be to pH 4. So logicaly interpreted the pH 4 calibration makes no sence for us, we calbrate to 4 and stay always at least 2 to 2,5 units away from it. Actualy to be most accurate we should calibrate secondly with a 9.6 buffer to be closer to the sample.. My personal pH in the high tech is pH 6.8
But as said this is nitpicking..
I guess the final truth is, we are wasting money on pH 4 calibration buffer..