The FB2900 introduces a 5th port in the form of an SFP cage.
SFPs (Small form-factor pluggable transceiver) come in a variety of types, the FB2900 will support up to 1.25 Gbit/s modules. The common supported types are:
A copper transceiver will add a RG45 1000Base-T port to complement the existing four copper ports, these are cheaply available and gives your FB2900 a total of 5 individual network ports plus a USB. The FB2900 needs the "NO RX_LOS" hardware type, which a majority are. This is down to how the SFP was wired with respect to the LOS Pin. This is thankfully the most common type. One final thing to bear in mind with a Copper SFP, is that they can only negotiate a link at 1G, so the device plugging into them MUST support Gigabit Ethernet. 10M and 100M are unsupported by the standard and will not get you a link.
Fibre transceivers are ideal for inter-building cabling and also if you have a fibre connection as the termination of your internet connection.
We have seen VDSL SFP modules, and these do work. We are a little wary of the build quality and heat dissipation on some of them. The FireBrick is able to use these and bring up PPP, but for the moment we'd still recommend a standalone VDSL bridge.
Most SFPs have a little ROM chip inside them with make/model etc, and most vendors use this to lock you into their own SFPs. The Firebrick 2900 is a bit more pleasant and actually doesn't really care what's in the EEPROM as such, so you are free to use /almost/ any SFP, provided the electrical connections are as expected.
Some SFP devices may emit invisible laser radiation. Read safety instructions provided by the SFP manufacturer.