What's the benefit of using an external audio recorder compared to an internal one?
There are a variety of reasons to have an external recorder depending on the size of your production.  A big advantage is the number of audio inputs and the ability to record each of those inputs as separate tracks, allowing you to do the mixing in post.  I'm not sure about the H4n, but I know the H5 and H6 support multiple inputs as well as interchangeable capsules.

Another advantage is quality and control over quality.  Most audio devices have a switch for uncompressed .WAV or compressed .MP3.  You always want uncompressed audio unless you are tight on space.  External audio devices also give you better control over gain and have better support for audio monitoring.

The final advantage I'll list here is portability.  When your mic is tied to your camera, you'll need to string cables from the mic to the camera.  This isn't an issue when the mic is sitting on the camera, but if you want a distance shot this becomes a problem.  External audio gives you much more freedom of movement to place your mic in the best position to pick up sound.