You will need an op-amp, a voltage reference and a AtoD converter.

Use a small resistor ( 22 ohms is good)in the current loop, feed it to the 2 inputs of the op-amp (with the usual series resistors, feedback etc.) Feed the equivalent voltage over the resistor at 4mA to the non-inverting pin (use an adjustable resistor I have yet to see a device that provide exactly 4mA at zero) and the output of the op-amp to th AtoD. I do not have a schematic at hand, but this is it. The 4-20 is so crummy that you will not gain anything by going beyond a 8 bit AtoD, so you might want to use a derivative with a built-in AtoD.

Have fun,

Erik

PS be ready for surprises, 4-20 is notorious for is drifting some in both span and zero.