Kicad is not backwards compatible. So it will not work without the danger of data loss!
- footprint libs will work in some cases. (as long as no footprint has new features in it.)
- pcb_new files might work. (I know that there now are new features for diff pairs. Not sure if you can open the pcb_new file as long as no diff pair features and no footprints with new features are used.)
- symbol libs need manual work (ensure no pin number is longer than 4 chars, then you can manually edit the header line to be version 2.3) If you do not do that kicad will wipe the information of the dcm file (or ignore it if you do not store changes to it)
- schematic files will not work at all. In kicad 5 the symbol lib is included to tell kicad from which lib a particular symbol needs to be taken. This removes the need for symbols to have a unique name over all libraries. In some cases it might work to manually remove the library names from the file format. But this is extremely dangerous and really not worth it!
- The project file also changed in some major ways. KiCad 4 for example stored the symbol libs that should be loaded in there. This is no longer the case in KiCad 5 as there now exist sym-lib-tables for that.
- The line order in files did also massively change. So if you intend on switching back and forth your version control history will look like an accident.
In general i would say if you know you need to work with somebody not capable of updating to version 5 then you should not work in version 5. Back-porting a project needs a lot of deep knowledge of the file formats. You would also need to take care while creating your project such that you do not use any new feature as that would make back-porting impossible without data loss.