This board has four 2.5G ports, not four 10G ports, so the 16G bandwidth for four PCie 2.0 lanes is more than enough.
You can't do line rate on all ports either (limited by PCIe alone, let alone CPU for smaller packets), but you can certainly fill an individual port, which I suspect is the goal.
Are you sure about that? With 5GT/s (or 500MB/s) per lane, and with 4 lanes, that should be plenty, no? Intel adapters like the x520-DA2 are specced at 2x 10G, and use PCIe 2.0 x8.
FWIW, I was also able to iperf3 around 3.7Gbps on a X520-DA2 connected to an RPi4's single-lane PCIe 2.0.
Ethernet speeds are given in data bits per second, while USB 3.0, PCIe 1.0 and PCIe 2.0 add in their speed values the extra bits used for encoding.
On the other hand, 10 Gb/s USB and 8 Gb/s PCIe 3.0 have really those speeds. Complicated :-(
One PCIe 2.0 lane is more than enough for 2.5 Gb/s Ethernet, but not enough for 5 Gb/s Ethernet or 10 Gb/s Ethernet.
Yah, I said forwarding-- so each packet goes over the PCIe link twice.
So, to use your test-- 3.7 x 4 / 2 = 7.4gbps. I said around 7. 7.4gbps would be a bit more than I'd expect, but I'd not be shocked.