Midea mini fridge dial 1 to 6, which number actually runs coldest?

5 is where I set mine. 6 would make my milk freeze against the back wall of the fridge in a day. 4 didnt hold ice cubes together in the summer when it was hot outside.
 
keeping the fridge at 6 all the time wastes less energy on these little appliances. The compressor for the fridge is only around 60 to 80 watts. Even with the added runtime at 50% it would still only cost a few dollars a year to run it at 6 instead of 4.
 
OP makes a point about the fridge dials on these Midea mini refrigerators not having any labeling like the other brands. It would have cost them nothing to put something like snowflake icons on the dials like all the other brands have so what a cheap design decision from them.
 
another question, is it possible for the main compartment of the fridge to be at 4C while the freezer box is also solid?

yes but its always a compromise on mini refrigirators with a single zone of cold air. The freezer box is closer to the evaporator so it will be colder than the main fridge compartment. You cant control both temperatures with these mini fridge units.