Hi Brycielkai,
PhysX SDK 3.4 has not been publicly released and from what I understand Flex is still a ways out.
PhysX recently released the 3.3.2 SDK which will be implemented in the coming releases.
As with a lot of Nvidia’s tech it is only available for their graphics cards thus eliminating part of the community from developing with it that don’t use their cards. If you would like to use their tech specifically you would need to license it from them and integrate the tech into your source build of UE4.
Thank you!
Tim