Proxmox v4 Beta as of right now uses linux kernel v4. I updated my homelab to it recently & PCI passthrough (per my previous post) stopped working. Asked on the Proxmox forums & apparently an additional option is required due to the newer v4 kernel:
Compared to what was required before:
Apparently vfio is the way to go with v4 kernels. It should be the default option once Proxmox v4 goes GM.
This is the default in v4 GM, so the option is no longer needed.