PHPoC basically runs scripts when it boots up. Therefore, it is possible that a P4M-400 cannot be escaped from infinite reboot when script contains system command such as "reboot". Refer to the Following to solve this problem since it needs to stop the running script.
Entering ISP mode
Make P4M-400 to enter ISP mode by supplying power while inputting LOW to ISP# pin. In the ISP mode, you can access to PHPoC by PHPoC Debugger without running a script.
Connect to PHPoC
Connect a PC to PHPoC via a USB cable and connect to the port via PHPoC Debugger. A message window related with ISP mode will be popped up.
Reboot PHPoC by using "Reboot a product" menu in PHPoC Debugger. After rebooting, PHPoC stops running script even it is not in the ISP mode.
Correct source code
Correct the source code to prevent infinite reboot state.