Four power supplies in parallel to a bacon light?
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Complete electrical isolation between machines can be achieved using relays.

Pick the machine with the most reliable supply for the bacon light. 

Add parallel relay contacts to the switch of the selected machine and feed the relay coils from each of the other machines. If the other machines have bacon lights they can be left in place, if required.