Maybe I´ve misunderstood you.
My opinion was you have played BR4 yet and now you want to "update" to the recent BR15.
You´re right - it´s not really an installation (starting an install.exe or similar), you only have to extract the compressed .7z file into a self created game main folder.
And the new BR15 you shall NOT extract into your existing game folder of BR4 but create a new folder for the new game and extract there.
Now you have 2 games: your old BR4 with all your character cards, clothings, housings and saves you have collected yet.
The other BR15 is a "clean" new game with only the content coming with the Repack. So you have to copy your game progress of BR4 into the new BR15.
This is easy in Illusion games cuz all this stuff is included in the "UserData" folder.
The BR4.zip you don´t need any more, the BR15.zip you can store (maybe extern) after "install" (I mean extracting) in the case of needing a new start if something goes wrong.
I'm currently playing the BR4, Actually I finished the game and just playing around with the states and actions etc... so maybe I'll be wasting my time with BR15
Anyway
That simplifies things, hopefully it works without any trouble
Thanks again!