4..20mA are current loop interfaces.
The complete interface only makes sense when the desired output current is independent of voltage (in a wide range).
It seems that the OP is not that experieced, so please be so kind to reccomend only circuits meet 4..20 mA standards.
All the circuits in posts 10, 16, 18, 20, 27 don't fit to a 4..20 mA interface because the are about proportional voltage dependent.
The circuits have about the same function as a simple pot.