Here I drew it up how I would of set it up.. Turn off lan dhcp on your old router, put its IP address in the address space that 4G hotspot hands out, same with your client bridge.. So you can manage its connection to the 4g, etc.
edit: client bridge in 4g hot spot?? Does the current sdsl router have wireless? I doubt the hotspot would support a client bridge mode connection to some other wireless network.
You would either have to use your current router and more than likely disconnect it from the sdsl network, or get a new router that support client bridge mode - routers that run 3rd party would be best choice I know dd-wrt supports client bridge, and I believe tomato does as well.
Or you could just buy a game adapter device (client bridge) -- they are designed to connect wired to existing wireless network, so that should work perfect, if they allow for more than 1 wired client.
If I remove the gateway for the wired will then both connections then work?
What is your current wired network?