LiPo Shim enable pin voltage range

I am looking at powering up a Zero for about 10 minutes via the LiPo shim once a day. The day timer is a PIC 16F628 jumped straight across the battery, so a voltage range of 4.2V to 3.4V. I will detect the low battery warning of the LiPo on GPIO4 with the PIC, send it to sleep mode and thus not try and start the Pi after the shim has pulled GPIO4 low.

What voltage range does the LiPo Shim EN pin accept and what is its min VIH? Is it 5V tolerant, or do I need a diode to keep the 4.2V out, presumably it has a pull-up resistor on board?