Here is a possible solution using only conventional components:
Use an opamp - as an adder. Remember that the current through the feedback resistor will be the sum of the current through the input resistors.
Having t.ex this adder circuit:
**broken link removed**
Then say all resistors is equal to 1kΩ, just for easy.
Step1:
You wish to start with 4mA. That means we have to make it go 4mA through the second input resistor. Putting 4 volts as V2 will solve that. You can of course change values of both V2 and R2 as long as it flows 4mA through R2.
Step2:
You need to know how much input current through R1 that will give 20mA out. Easy - as said before, output current is sum of input currents. You already have 4mA now, so that means you have to get the others 16mA's from current through R1:
So - When you put 1 volt over R1, there should go 16mA through it. Ohms law then give resistor value to be R1 = 1V/16mA = 62,5Ω. You will need that exact value. Using E12 resistors that is 68Ω||820Ω||15kΩ or simply use a potmeter.
Step3:
You can remove the Rf resistor. The current you wanted will now go through any resistor (or cable) that you put in place of Rf.