I've been following the issue CM4 is missing IEEE1588-2008 support through BCM54210PE since I heard about IEEE1588-2008 support on the Compute Module 4 last year.
Apparently the little NIC included on every Compute Module 4—the BCM54210PE, which is different than the NIC on the Pi 4 model B (a BCM54213PE)—includes support for a feature called PTP, or Precision Time Protocol.
Ahmad Byagowi, the project lead for Open Compute Project's Time Appliance, reached out to me a couple weeks ago and asked if I'd be willing to test the new Time Card Facebook had announced in mid-August on a Raspberry Pi Compute Module 4. Since I have a sort of obsession with plugging anything and everything into a Pi to see what works and what doesn't, I took him up on the offer.
The official specs had PCI Express Gen 3 on a x4 slot as a requirement, but it seems the Gen 3 designation is a little loose—the card and its driver should work fine on an older Gen 2 bus—like the one the Raspberry Pi Compute Module 4 exposes if you use the official IO Board:
The slot is x1, but you can plug in any width card using an adapter like this one or by hacking an open end into it with a razor saw or dremel tool.