I think there's only one D7800 model, X4S might be an extra bit to indicate some wifi capability (like the R7800), the D7800 has X4S, 100UKS might indicate its power supply has a UK plug, or cables with plugs for UK phone sockets.
Yes its VDSL2 modem is Lantiq chips (VRX320). The rest of the chips, CPU, Ethernet, WiFi etc., are Qualcomm Atheros.