A question about replacing voltage outputs with 4-20mA current outputs
A current signal as opposed to a voltage signal isn't so much to reduce environmental noise pickup, but to eliminate errors caused by ground offsets and the series resistance of the cable.  Current loop can be lower noise, depending on your noise sources, but there are also scenarios where a voltage signal can be lower noise.

There are other ways to transmit signals with little noise pickup, like balanced differential voltage in a shielded cable.

However, for the best noise immunity, send the information digitally.  Put a micro as close to the source of the signal as possible.  Have that sample it digitally, then send the information on.

With digital, you can guarantee correct reception as long as some minimum signal to noise ratio is met.  You can also include a checksum, so that if data is corrupted, the receiver can know it is corrupted and deal with it accordingly.  You can also send multiple signals over the same channel quite easily.

Unless you have very high bandwidth requirements, it is usually best to convert to digital as early in the process as possible, then do everything digitally from there on.