I have a h4n pro and a mix-pre 6. As @marcus_fischer said, the h4n is totally fine for a lot of applications esp. for recording live music and field recordings of settings with sufficient volume. Even the built-in mics are fine again for subjects where you have sufficient volume. I use it a lot to just carry around and capture sounds when I don’t want to go to the trouble of having to set up a rig.
Where the mic-pre 6 and the sound devices in general are better: the preamps.They really shine on more delicate and quieter sounds because the preamps are significantly quieter than the ones on the h4n. They also respond to dynamics with a bit more flair. You need to buy external mics, etc. but you’ll notice a big difference in these kinds of recording settings: lower noise, clearer details, more accurate dynamic range. They also give you more options in terms of recording quality, etc. and the built-in limiter is better than the h4n. That said, I don’t know if most people even dig into these features anyway.