There are two main issues to consider when dealing with your project :
Let's say you want to record continuous 24 hours audio on two tracks and that every 24 hours you can stop recording for a few minutes to change recording medium and power supply.
Although you mention that
Audio quality is not extremely critical
I will suggest to record in PCM format at 44.1 KHz or 48 KHz sampling frequency. This implies that a
.wav file cannot handle such duration as it has a file size limit that is under your needs.
24 hours stereo 16 bits in 48 KHz is a little under 16 Gb.
Fortunately, many recorders have a functionality that will split the recording in several files if the file size grows over the format limit. It means that recording 24 hours will leave you with several files that you need to concatenate to obtain the full duration. It seems that zoom H4n and H4n pro offer this functionality. You need to check that by yourself as I couldn't find a definitive answer in the product's guide.
So, you need a 32 Gb recording medium to be on the safe zone.
The external power supply available for zoom recorders mentions 5 V / 1 A. If a power plug is present, use the external power supply. If it is not, for 24 hours recording without interruption it would require a power supply of 5 V voltage with approximately a 24 A.h charge. (And you would need one of these for each 24 hours time slice).
Notice that the nominal charge of a given accumulator may vary a lot depending on the technology used and the actual temperature. (I mention that because of the siberian mention on your name, in cold temperature, expect a much lower capacity than nominal one).