The most likely place is the LAN default rule. The default rule states that LAN subnet is allowed out. 192.168.4.6 (and .5) is not on the LAN subnet, so it is blocked. If you have adjusted the LAN rules to allow your second subnet, then I would look at changing from ProxyARP to either CARP or IP Alias. I have never used proxyarp as a gateway, so I don't really know. Also, is the default gateway on 192.168.4.6 (web server) the pfsense proxyarp? if not, then you are creating a routing problem.