What is the difference between 2-wire and 4-wire transmitters?

2-wire (loop-powered) transmitters: power and signal use same two wires, transmitter draws 4-20 mA from loop, limited power available for transmitter electronics, simpler wiring, lower cost. 4-wire transmitters: separate power supply wires (24VDC or AC) and signal wires, more power available for complex electronics (smart transmitters, displays), can source larger current for multiple loads. Selection depends on transmitter power requirements, installation distance, and system design. Most modern process transmitters are 2-wire, while some specialty instruments require 4-wire.