Input is insufficient

The loop testing procedure for a non-HART instrument isn't any different than for a HART instrument. The only thing that you might need to add is a 4-20 mA generator in place of the instrument to simulate the full range for testing your control system inputs, HMI, etc. Some non-HART instruments can do this from a menu internal to the instrument via a keypad so you don't even need the current transmitter.

Depends on the circuit. Say its a loop powered inst then may be a resistor of the right ohms in place of the inst will do.

v=ir ..

if its not a loop powered and then you might need to use a 4-20mA generator in place of the instrument,

Hope this helps.

The OP stated that it was a 4-wire instrument so it isn't loop powered.