4-20 mA Signal Booster

Look at the Lee Dickens (usual disclaimer) range, among others. 4-20mA repeaters are in their range. Clearly, each repeater requires a separate power supply.

Consider also raising the loop supply voltage so that a repeater isn't needed. <...quite far...> 1km on 4-20mA in 1.5mm² cable isn't usually a problem on a 24V supply, though it depends on how much stuff the transmitter is driving. Instrument manufacturers usually give a graph showing the maximum loop resistance for various supply voltages for their transmitters to work correctly. It's all in the manual and a few tests with a multimeter can soon indicate the correct way to go.

The beauty of 4-20mA is that they're more immune to noise and reach far longer distances than voltage signals.

As long as the loop resistance doesn't exced 900 ohms for a 24 V power supply (just an example), it doesn't care what the voltage drop is, because it is not monitoring voltage, it is a controlled current source.

The number of devices you feed this signal into, is not important either, as long as they don't comprise a series resistor of more than the maximum across which, a 24 V power supply would create a minimum of 20 mA current (use Ohm's law).

I know I said that is more immune to noise than voltage signals, but is not absolutely immune, it still can pick up noise, to diminish this use twisted pair wires with a shield or foil shielding, and ground it only at one end.

You can have a closed loop of, say 3000 feet with no repeater at all, just do your homework (investigate more).

Regards