For what you want to do, you need to either make a custom cable or use the passive hub.
Custom cable:
You need several pieces of Ethernet cable, one short (~6 inches) and the other pieces long enough to make it to where you plan to have the SSCs on your horse. Strip back about 6 inches of the outer insulation on each piece of wire. Untwist enough of the orange-white/orange pair to strip a half inch and connect all of the orange wires together as one group and all of the white/orange wires together as a separate group. Do not untwist more than a couple of inches if at all possible. Cover the connections with electrical tape or heat shrink tubing.
The short one plugs into the dongle, you can trim off the other 6 wires as they are not needed. For the other cables, untwist the other 6 wires and group for ground and +12 v. The colors change depending on if you have an "a" or a "b" style cable, so it would be best to use an ohmmeter to determine which are the ground wires and which are the +12v by attaching a ssc to the cable.
Strip about an inch from the six wires, group, and hook each set of three to one of the little power converter/regulators that you were planning to use to power each ssc separately.
You can probably get away with 4 SSCs like this. However, there is a catch... Any unused cable ends need to be terminated with a 120 ohm resistor between the orange and white/orange wires. No termination is very bad...