The Neptune 4 is equipped with an inductive probe that triggers from the metal build plate and measures the distance from the probe to the plate which in turn is translated to a Z distance of the nozzle above the bed. Because of this operation without the plate or trying to use glass or garolite plates can result in nozzle crashes.

The probe is highly accurate with a precision in fractions of a micron.

It's located in the left rear of the print head and above the nozzle. A small red LED indicates its operation.

Note that the probe position is different, or offset, from the nozzle and so when it's probing it's the probe that's over the probed position not the nozzle.