pert:
When you press the reset button, does the LED start pulsing?
You were supposed to change the port in the Arduino IDE's Tools > Port menu, not Device Manager.
Do you get the "Caused by: jssc.SerialPortException: Port name - COM4; Method name - openPort(); Exception type - Port busy." error message, but with the bootloader's COM port instead of COM4?
Yes, the LED is smooth pulsing. And yes, of course I change the COM port in the IDE. The result is the same, only with the other port number. It does not work anyway.
It looks like the driver is blocking the COM port. It does not work synonymous, the serial monitor, because the port is busy.