However, I think that this will only share the bandwidth load between the four WAPs........... by this I mean the Monowall will only see for clients on the network.
Unless you have configured your WAPs to use a NAT for it's clients a WAP behaves the same as a switch or hub.
This is very easy to check: Do you have access to network shares from one PC behind a WAP to a PC behind a different WAP? If yes m0n0wall will handle clients behind your WAPs individually.