Since nobody knows what app your wife will use for reading and if she needs the frontlight and/or Wifi, Bluetooth et cetera no on can really say how often a device has to be charged with 3-4 hours reading per day.
Also since it's Android there are usually some apps always active in the background (like the shitty Google apps). You can deactivate (or maybe deinstall) them via settings or adb if you don't need them. Plus you could use Saver Tuner:
https://f-droid.org/packages/s1m.savertuner/ (although this requires an adb command to run).