Honestly, it's just going to be the Navigation that will be your deciding factor.
When using Nav, If you are in an area that has poor cellphone reception, you'd be better off with the 8400. Otherwise, just plug into (or use the wireless) Apple Carplay! Everything is handsfree. For whatever reason, Pioneer doesn't voice-nav anymore....meh...
If you ever change your mind about the 4400, you can just purchase the add-on nav unit (AVIC-U280)! (retail price is rather outrageous however).