You will see 5v across Ron's 250 ohm resistor at 20mA input. So that give 2.5v into the op amp. If you eliminate the wto 100k voltage divider, and connect the (+) op amp input to the 4-20mA source, then you will get a 5v dynamic range.
if you change Ron's resistor to 300 ohms, you will get 6v range at 20mA to match the 6.2v zener on the (-) op amp input.
Also, R6 is way too big. That can be cranked down to 10 to 1k ohms. No current flows through that once the gate is charged (and a 100k resistor will take a relatively long time to charge the gate.