Firewall/NAT

Hi-

Looking for a little help with configuration of a new m0n0wall installation...

Setup is,
interface #1 wan (public ip)
interface #2 private lan (private ip)
interface #3 private wireless (private ip)
interface #4 public wireless (private ip)

My only hang up is, configuring the Public Wireless interface (#4). I want to configure the rules on that interface to allow access to the WAN only and not allow for access to any other network.

So far, I've created several rules on this interface-

1. deny access from Public Wireless to Private LAN
2. deny access from Public Wireless to Private Wireless
3. allow Private Wireless to access *

Question is- is this the right way to do this? I would have preferred to create a single rule allowing access from Public Wireless to "WAN" and rely on the implicit deny to block everything else, but I see that isn't an option in the ruleset? I don't see a "WAN" interface, only WAN address.

Any suggestions?