Hardware

Standard the Soekris Net4501 has 3 Natsemi NIC's.

Since I have one PCI NIC, I thought of adding this to the soekris. More physical NIC's for more (low-traffic) networks.

Can M0n0wall (embedded) work with 4 NIC's (one intel Pro/100)?
If I'm correct 4 networks should not be a problem (some Alix has 4 NIC's and some newer Soekris also).
And Intel should work, only I haven't seen it work yet with M0n0 (it did work with some custom Linux-system for a short while).

Trying getting a card Soekris they have 2-port and 4-port sisX cards that will work just fine.  Although I don't think a 4-port will fit in a 4501 case.

Adding a 4th nic should be no problem. It all depends on the size of the card.
Don't know if you have the soekris chassi. Then you might have to cut a hole.

I added a Wifi PCI card to my 4501.