Arduino Raspberry Pi cannot restart
According to this thread on the Raspberry Pi forums, it is a known issue that the Pi 4 will not boot if power is present on its USB port.

There is some evidence that switching to one of the USB 2.0 ports (rather than the 3.0 ports) solves the problem. EDIT: OP tested this and it doesn't solve the problem.

You can also get into the USB cable between your Pi and Arduino and clip the red power wire (if you don't need to provide power to the Arduino through this USB connection).

Older versions of the Raspberry Pi are not affected by this.