PLC to act as or simulate 4-20 mA transducer
What I believe you need here is a DAC to give 4-20mA output to feed your 4-20m4 inputs. 

The more flexible and expensive solution is a real DAC. Usually 0-10V. Simply drive a current amplifier scaled to give a maximum 20mA. The analogue outputs of PLC can sometimes be configured as current sources as well.

The cheaper way is to use a voltage source and series resistors switched by some relays. Unless there is some compelling reason I would use 3 relays and resistors to give me 4mA, 12mA and 20mA sources. (Bottom, Middle and Top of the analogue range to be tested). This catches most issues I've seen in production. The relays can be driven by any suitable digital output of a PLC.