Be aware the experimental interface is now on version Chaos in the latest branch.
Aperion and now Chaos are just part of the interface change over.
The release notes kind of tell you why and what the interface is, and at the bottom sort of explain why there’s an experimental folder.
When physx 3.4 came out, it had already been available in the UE4 source for months.
Unfortunately I don’t see anything glaring in the source that says 4.0 is happening soon.
But it’s possible the interface was built with 4.0 in mind. And it’s probably not a coincidence that the interface came out and then 4.0 was announced.
We just have to wait and see what happens.