Using Gentoo might work if you enable the KDE overlay: https://wiki.gentoo.org/wiki/KDE/Ebuild_repository
Gentoo supports slots; which allows for concurrent installations of things like desktop environments. Not sure if it’s configured that way right now though.
If you use BTRFS you could install gentoo on a subvolume and boot into it when needed too.
I’ve only recently moved back but I was using it before the KDE4 release and you could do it with 3.x. Shame