Do you have 3 extra wires? Doesnt RS485 typically have a twisted pair each for transmit and receive? In any case, it depends on the distance and your current draw. If it is CAT5 (ethernet) or phone cable, you will have quite a bit of voltage drop from the host to each slave device that is distance/draw dependent.. You could perhaps use AC rather than DC, and just have some simple rectifier circuit on your slave devices. If you have a larger gauge wire, you will be better off, but still might not work.