There is nothing in the installation of LR 6 that will affect or delete your LR 4 catalogs. Just go ahead and install it.
The first time LR 6 opens, it will ask if you want to upgrade your LR 4 catalogs. You should say YES or OK or whatever the affirmative choice is.
So just to be clear, please bear with me here, (with existing LR4 catalogues backed up) do I un-install LR4 before installing LR6?
Or do the two versions merge when I agree to upgrade LR4 catalogues?
The two versions are independent of one another. You can uninstall LR4, or not uninstall LR4, Lightroom 6 doesn't care.
I just don't want both versions on my machine, it's a waste of space and there's no point really.