I bought a Soekris net4801 before I realised m0n0wall didn't support it yet
So after reading the Hackers Guide and the following generic FreeBSD guide
on the net4801:
I now have m0n0wall running on my net4801.
It is based on the pb18r522 image running on a 4.8 kernel and can be
downloaded from here if anyone is interested.
I installed this using Manuel's physdiskwrite tool.
Not sure if I got it right, but it seems to work so far :-)