For routing and firewalling between 4 L2 networks, I currently use a WRT54GL (without using the WIFI). There are 4 networks routed which come in by VLAN from a managed switch, so a only a single Ethernet plug is needed. This solution works great: It is cheap, low-power, easy to add hot-standby devices (because it is cheap). The performance is not the best but it does its job.
Howver, there is not much memory: Tools such tcpdump would be nice to facilitate debugging or OpenVPN (which currently run a server behind). Also IPv6 is not very mature on OpenWRT.
Raspberry Pi would need similar power, is cheap as well but there is not memory drawback. It has only one ethernet plug, however, due to VLAN, this would not be a problem.
However, I am not sure if the Raspberry could provide a similar performance and stability (on the WRT, I have several hundred days of uptime!).
Anybone played around with the pi and can tell if this would work?