This would be forward compatible. Backwards compatible would mean you can open a file made by v5 and you can open it in v4.
If kicad is forward compatible might be debatable. As opening a v4 schematic in v5 is not possible without remapping it.
Remapping means adding additional information such that kicad knows from which library each symbol comes from. (In kicad 4 the first lib with a symbol of a given name was chosen. This meant symbol names needed to be unique over all libs or the user needed to take care of library priorities.)
Yes. But depending on what your operating system is this can get quite involved.
In short you would need to set the environment variable XDG_CONFIG_HOME of your v5 installation such that it uses a different config storage location than v4.
More details see:
Another option is using a virtual machine setup for running the version of kicad you do not use that often.