The PLC has a 0-20mA Output, and the input of the other device is 4-20mA?
In that case you should scale your output between 0-16mA and then add 4mA.
Let's assume that it's a 12bit analog output module. Then
0mA = 0
20mA = 4095 ---> 16mA = 3276 AND 4mA = 819
To make the calculations for the output, work with the range 0 - 3276, and the add 819 before setting the analog output. (You cannot work with 819 - 4095 directly, because thats not a linear function) That way, when you need a 0, you are setting a 819 that is 4mA and your rectifier will understand it as a "0", and the same for the upper limit.
I hope you understand what I'm trying to explain. Good luck!