Half-related question: the chip on the board looks like an STM32 microcontroller, wouldn't a 6-pin 8-bit PIC10 or similar µC be sufficient and cheaper for the purpose? And possibly use less power, a PIC10F322 in sleep mode with the watchdog timer enabled is around 0.5 µA, while a small STM32 is more in the 100 µA range in the best case.
I was wondering why an MCU with so many pins was used. Maybe it was picked for the total package height?
Probably. Or something like the TPL5110 might actually suffice, purpose-built timer for power gating at 35nA, but only goes up to 2hr at max.
Nice, didn't know this one! This combined with a PIC10 without watchdog timer might also be an interesting combination to do more complex things. Or perhaps simply a TPL5110 with a ultra-low-power flip-flop (does it even exist?) used as a divider to get to 4 hours.