The problem *shouldn't* be related to the change of security. If the Wireless Network Adapter doesn't support WPA, it won't be able to connect but it should still find the network.
You don't say which version of Windows you're using but, in the connection's Properties, there should be an option to Connect even if the Router isn't broadcasting its SSID. Make sure that option is enabled (even though the SSID is being broadcast).
The number of connections to the Router *isn't* restricted to four just because there are only four physical ethernet ports.