When entering an IP address in the Custom page of the connection dialog in 4D Client, what IP address should you use?

The answer is simple: any IP address (primary or secondary) that is valid for the machine 4D Server is running on. 4D Server can reply for any valid IP address for the machine. Please note that when it comes to Web serving or Web services, the IP addresses for which 4D Server replies can be restricted through the preferences.