industrial signalling and noise immunity

I deal with alot of industrial 4-20mA. the cabling from tranducer/ transmitter to output is is always twisted pair, shield c/w a drain wire.

1. twisted pair is so induced current signals (from magnetic fields) cancel, correct?
2. shielding (foil etc), acts a faraday cage to the wiring within essentially 'shielding' the charges in the signal wire copper from outside e-fields influence, correct?
3. what is the drain wire for?
4. would there be negative outcome with using an overall shielded triad cable c/w drain wire, isolating one wire, in the stead of the twisted pair (ie if the contractor accidently pulled triad and not twisted pair)?