Multi-port WAN firewall

Here's a new question I've never been able to answer.

I'm trying to build a multi-WAN Linux router. Normally, I have four WAN
ports:

WAN1: Cable modem (fast link)
WAN2: DSL line (slow link)
WAN3: Public WiFi
WAN4: DMZ Servers
LAN1: Connection to Internal network

The box has a built-in GigE connection for the LAN side, and since all
four of the WAN connections are less than 100Mb (typically less than
10), my first thought would be to find a 4-port PCI Ethernet card. These
are, however, very hard to find.

I could also uise a few NAT routers, but this could be quite complicated.

Finally, I could do something like this:

1. Get a VLAN-aware switch
2. WAN gets VLAN tagged with four tags (WAN1, WAN2, WAN3, and WAN4)
3. LAN stays as in example 1
4. I plug all of the WAN links into the switch and do port tagging for
each one.

Which one would people do?