Using the new KiCad 5 library with KiCad version 4
As stated above the new lib is allowed to contain footprints with pad shapes incompatible with version 4. The library team did however release a backport for the v5.0.0 library which is compatible with version 4 (it has a slightly reduced number of footprints) https://github.com/KiCad/kicad-footprints/releases/tag/v4-backport-5.0.0
For any other version of the new official library (or for third party libs) one would need to create the backport manually. (Later versions of the version 5 library will not be so easily backported as rounded rectangle pads will be used in more footprints.)
To be able to at least use all valid footprints one can remove (or manually edit) incompatible files.
A script that lists and optionally removes all incompatible files has been developed: check_kicad4_incompatible
Sadly the script right now does not check for the new 3d model offset field. So some manual work might be required if you have 3d models with offset different to 0,0,0. (0 offset is stored backwards compatible by kicad. The official lib does not allow offsets differing to this so we have no need to include this in the script)