Voltage source & current input
You can use an opamp and a potentiometer like this:







simulate this circuit &ndash; Schematic created using CircuitLab

Whatever voltage you set by adjusting R2 will cause the opamp to source current to produce the same voltage drop across R1.

Since 5/250 = 0.020, this circuit should be good for a 0 - 20 mA range.  You can cap the bottom end at 4 mA if you add a series resistor to the potentiometer.  I will leave calculating that to the reader.