@pcarmo dev/ttyS4 are the UART pins on the 40 pin header, the 4 pin header we haven’t worked out what it is yet - see this thread:.
As far as a UPS I would suggest using a USB-Serial dongle with an FTDI chipset that would provide the correct RS232 voltages to the UPS. don’t forget, the standard RS232 is +/- 12 volts and the header pins are at TTL login (+/- 5v) so you can damage them if you hook them up directly.