It's must either be more than one NIC on the same PCI interface (useful to save space on the motherboard) or a PCI TAP device, used for sending a copy of the NIC's traffic somewhere else for auditing purposes.
We have several of these 4-port Ethernet cards in firewalls. The card can either act as four Ethernet NICs or as a 4-port Ethernet switch internal to the server, depending on how you install it.
Another possibility, more likely on high-end kit, is that they provide extra console (serial) and administrative ports that also use RJ45s. I've seen this on some Solaris boxes but may be on other servers too