Here's a nice application note about wiring up RS485
What I am suggesting is using 8 conductor CAT5 wiring.
Use 1 twisted pair for +5/Gnd
Use 2nd twisted pair for +5/Gnd
Use 1 twisted pair for data from near end to far end.
Use 1 twisted pair for data from far end to near end.
At the far end, receive a message from the near for switching the LEDs on/pff.
At the far end, sample the sensors and send messages to the near end with whatever they are telling you.
Use a board with some IO so you can connect everything.
You can use this board too
Only does half-duplex vs full duplex.
The app note explains about that. Connect up the unused pins to bring power down the line.
Your next post = 1000's of sensors.
These will all go back to the same 'master node' ?
So 50+ slave nodes with 20+ sendors on each one?
I think you might want a custom board for the remote nodes for that.
Maybe use inexpensive 434 MHz RF transceivers (nRFL2401+ based) at each end. Have the master poll each remote sequentially to get their data and send LED commands.
Post a link to the HC04 sensor, lets see what's involved to use it.