Drive LEDs from 4-20mA source
I believe you can simply connect all four LED's in series (observe correct polarity) with the 250 ohm resistor. The 4-20mA transmitter will take care adjusting the current/voltage for you. Just be sure the LED's rated average current is around 20mA, that way the LED's are fully lit at 20mA and dim at 4mA.

*EDIT - The 4-20mA transmitter should have a specification called "load range", your resistor should somewhere in this range.