If 4...20mA current source is connected to more than only one shunt resistor, instrumentation amplifier design will still show good performance. Have a look at a more detailed schematic (but it's still a simplified schematic!):



You see, that now 4 different signal grounds are distinguished.
Equally which signal interfacing is used, either symmetrical signal routing like RS-485, or the use of instrumentation amplifier or finally the use of galvanic isolating optocoupler, it's always neccessary to keep common mode noise, means potential difference between all involved signal grounds as small as possible. Either, because common mode range of used methodes is always limited to some maximum, or because protection of barrier of optocoupler cannot be stressed arbitrarily.
The best methode to do this, is to connect each signal ground individually to earth potential at power supply unit. By the way, this connection is a must if requirements of 'safety class I' must be fullfilled!
In nearly all applications resulting ground noise when doing this is much smaller than 10V. Only the poorest grounded systems show bigger common mode noise. But then this is an indicator for a heavy grounding mistake and a saftey problem! In this case whole earthing concept must be carefully analyzed and mistake as soon as possible be fixed.
But it would be the wrongest way to remove the earth connections and to choose some optocoupler solution. Problem remains and becomes even worse!
If the focuse is on safety and suppression of high frequency EMI, it's also recommeded to connect different sections by cables having a shield, with shield connected at both ends to according signal ground, and by this with local earth potential.
Some people will say now, that ground noise will be increased, because additional ground loops are introduced. But in very most cases, just the opposite is observed. More ground loops means, that individual loop area becomes smaller and smaller, and by this loop noise heavily decreases. Also, if there is some current flowing through earth connections and causes a voltage drop, with more loops effective impedance heavily decreases and by this voltage drop.
So, making as many as possible earth connections, resulting in an earth grid or mesh, which can be considered as an idealized ground plane, is the best and ONLY way to drastically decrease ground noise.

Many people think, that ground noise is something God given, which they cannot influence. And because they fear to get problems with too heavy ground noise, they omit as many earth connections as possible. But that's totally wrong! Ground noise can heaviliy be influenced and the best way to reduce it, is just to make as many as possible earth connections. Do use every cable shield, with it's earth connections at both ends to make the earth grid more solid. The result can be really dramatical!
If this methode is combined with symmetrical signal routing and the use of big signals, you will never get any problems! But if you are anxious and remove these earth connections, situation becomes worse and worse. And even with the expensiviest measures problems will stay.

Use of solid earth grid has many more benefits, than I can talk about, here. It is advantegous in suppressing effects of cell phone radiation, suppressing damage potential of electrostatic charge and even when building is hit by lightning, earth grid will help to minmize potential difference between different signal grounds. Use of earth grid also stabilizes impedance of ground routing. You cannot reduce ground impedance over the whole spectrum of frequencies, from DC to cell phone radiation, more by any other methode. You cannot decrease trouble making resonances, caused by inductivity of ground connections, more and better by any other methode.

Kai