When you use the nVidia SDK - no matter if 2.8.3 or 2.8.4 - you must fulfil nVidia's SDK license conditions. Basically, you need to inform nVidia when your project is published, additionally to the other deployment conditions like displaying the nVidia logo on package and splash/credit screens.

Theoretically, all PhysX versions are supposed to be backwards compatible, so 2.8.4 should work.