Confirm whether you are replacing only the nozzle or the complete nozzle assembly. Elegoo publishes separate guides for both.
For the nozzle-only procedure, remove the front cover, heat as directed and hold the square heater block with pliers while loosening the nozzle with a 6 mm tool. This prevents torque from twisting the heatbreak. Hot metal can cause severe burns.
Install the correct M6-compatible part by Elegoo’s method, restore the cover and inspect heater and thermistor wires. Set the nozzle diameter in the slicer, then run leveling and a first-layer test.
Use the complete Neptune 4 and 4 Pro guide. Stop if plastic leaks above the block or temperature fluctuates.
If you’re also considering a different nozzle diameter or a swap to abrasion-resistant filament, the guides on nozzle sizing beyond the stock 0.4 mm and brass versus hardened steel nozzles cover what changes with each choice. Pair a fresh nozzle with the printer’s regular cleaning and inspection routine so problems don’t get mistaken for a bad install.