Stick with the .4mm for now. It will do the job. It’s the layer height, after all, where you will see your best gains in detail and clarity. The Ender-3 is rated at .1mm but I’ve been sending stuff to the printer at .08mm layer height.
Since you are relatively new to 3D printing, I can only recommend that you learn everything well using the .4mm nozzle. Once you are comfortable, then you can decide, based on the results of your many hours of printer, whether ot not it is worth getting the smaller nozzle.
"Nearly all men can stand adversity, but if you want to test a man's character, give him power."