Just side by side installation of V4 and V5 is possible using KICAD_CONFIG_HOME environment variable. So I would not expect nor do I see the need that developers will do anything about it
Serious problems arise with the libraries. If you will use the same libraries with V5 as with V4 you should not have much problems. But then again, if this is the case, you also should not have much problems upgrading your projects, making V4 obsolete for you.
If you want to use different libraries in V5 than what you are using with V4 then you will definitely have some issues. But this is really the case where you want to use V4 and V5 as is usually too much effort to upgrade V4 projects to use new libraries.
If either V4 or you set up system (or user) wide environment variables, pointing to footprint, 3D model, symbol libraries, and templates, obviously in V5 you will have to use different variable names. This makes it impossible to use KiCad V5 libraries and their
fp-lib-table directly as you will have to change embedded variable names.
The workaround is to remove system (user) wide definitions and move them inside KiCad V4 and V5, which makes it possible to use the same variable names in V5 pointing to different locations.