Technically, with a single 802.11n AirPort Extreme Base Station (AEBSn) plus one 802.11n AirPort Express Base Station (AXn), you would have the minimum to create four separate wireless networks, each having its own unique SSID. The AXn would need to be connected to the AEBSn by Ethernet. The AEBSn would provide three: 1) 2.4 GHz network, 2) 5 GHz network, & 3) Guest network, and the AXn would provide one.
Physically you can connect up to three AirPort Express Base Stations to the available LAN ports on a single AirPort Extreme. With an Ethernet switch, you can add even more Express Base Stations.
The issue will be your DHCP server. By default, unless in bridge mode, the AirPorts all have built-in DHCP servers. In your proposed configuration, you can configure all of the base stations as bridges, but you will have to figure out how you will be providing NAT service for your local network.
For the AirPorts, the Connection Sharing setting (in the AirPort Utility) controls both the NAT & DHCP services provided by the base station. You only have three choices: 1) Share a single IP address = NAT & DHCP enabled, 2) Distribute a range of IP addresses = NAT disabled / DHCP enabled, or 3) Off (Bridge Mode) = NAT & DHCP disabled.