Below is a link to my test, see Trial #2:
Link
The graph below is a detailed graph for the 3xAA's End-Of-Life Only, not the full test.
4.20 Volts (or maybe even as high as 4.4V) could be the Alarm Threshold, this depends on your duty-cycle and additional loads.
When discharged to ~3.4 Volts, the battery pack could not meet the Peak Current Demands of the Boron LTE during Startup, and the test was terminated.
Below is the full test graph. As you can see, the Voltage bounces a little during the test. This was directly related to the Ambient Air Temperature in my office (because Heat is Energy).
L91's Operating Temp: -40°C to 60°C
Note: These tests were performed w/ a previous Firmware version & at room temperature, but might give you a starting point on what to expect.