Cloning a USB signal - feed two USB hubs with one input port
You can't duplicate the signal because, ignoring the electrical challenges, the protocol does not support it. The USB host would not be able to understand who it is talking to if two slave devices were talking back to it at the same time.

To put it another way, why do you think we have USB hub ICs in the first place? If you could just share the signals between multiple devices there would be no need for a hub.

If you have two devices (e.g. two hubs) which need to share the same host port, you simply need another 2-port (or more) hub IC to connect them. So using 3 of the 4-port hub ICs you could make your 8 usable ports - in fact you would have 10 usable ports, 2 spare on the first hub, and then 4 on each of the other hubs.