Not fair to use mouse and Keyboard

The xbox one has mouse and keyboard support so it’s all fair, plus many gamers struggle with controllers and find it way easier using a mouse and keyboard.

For the following to work you must make the menu selections using your keyboard. If you use the controller to make your menu selections to start the game you will be placed in a room for those with a controller and your keyboard/mouse will be non functional
to make it fair to other players. To enter the room for keyboard and mouse users the game must be started by selecting the options using the keyboard/mouse. Hope this helps.

Mouse and keyboard support

Fortnite on Xbox One supports the ability to play with a mouse and keyboard. Note that when using a mouse and keyboard on Xbox One, you’ll be placed in a game with other users who also chose mouse and keyboard as their control option. To enable your setup
to use mouse and keyboard, see Mouse and Keyboard Support

Note If you’re on a child account, you’ll also need to enable settings for cross-network play to use a keyboard and mouse while playing
on Xbox One.

Is nice, but then again, if those were PC gamers, I would not find it a fair game as they use keyboard and mouse.

Due to it being a competitive game I would say No.

The right way to Support Keyboard and Mouse on console would be to have a Keyboard and Mouse Playlist that is cross play with PC, this would then mean the playlist would be active and fair.

Ok, so firstly you should be able to just plug in the mouse/keyboard USB and use it (if they are supported) you shouln't need an adapter to use mouse and keyboard on Xbox One since mouse and keyboard support has been added.

Secondly I'm not aware of black ops 4 supporting mouse and keyboard use on console, so if the developers have not enabled mouse and keyboard support on Xbox One, then using an adapter to be able to use your mouse and keyboard will be considered cheating.