Arduino IDE remains connected to the same port
...if Monitor or Plotter is opened.
In whole - correct, it needs:
1) Make sure that the USB-UART bridge IC (of your board) driver is installed
2) Device manager of Windows OS should show it among COM-ports, when plugged in (without any yellow marks).
3) Any serial port monitor (one among all: B4R IDE, Monitor of Arduino IDE, any other app) can open the port and occupy it exclusively (other apps could not open it while it is occupied)
4) B4R IDE's Board selector is the final interface to select the port before compiling the B4R sketch.