Transistor for 4-20 mA voltage drop resistor connection?
Using a resistor in series with a 4- 20 ma signal is common practice to measure the voltage drop.

Something like a 300 ohm resistor will give you a 1.2 V signal at 4 ma and 6 V at 20 ma.

Can you provide more description of how you are using the transistor?  Possibly provide a circuit diagram.  It sounds as if you are trying to get the transistor to switch at a discrete signal level.