Firstly most of you seem to have very limited experience with 4-20mA instruments. They work wonderfully not only the ones I make but dozens of other companies I have commissioned at site. Good resolution, stability etc.
Now for the soln, If the signal source is already there(field instrus) then a quick soln is using a loop converter ic like RCV420 etc for conv 4-20mA to V, go to Burr Brown. The output can be tailored for most ADCs. If you are up to some design then a resistor with protection circuitry upstream and any dual slope/ SD adc will suffice, you could even use the onboard ADC of some uP. There are even some modules that do all this and give serial data out.