The 4400 does everything I need.  Everyone says the screen is more responsive on the 8400, but I didn't notice any difference - I DO see a brightness/saturation upgrade on the 8400 though.  It has a glossy feel to it which looks way better IMHO.  I tried tinkering with the GPS and man it was like I went back in time and was trying to use a garmin.  Is there any voice input method?  Or do you have to type in the city first, then the address and hope it has the address built in and lets you type the whole thing?  One of my doctor's buildings is only a year old and their address didn't show up and so it wouldn't even let me complete typing in the address (grays out the letters)... I typically use Apple Maps so my apple watch dings when I need to turn, etc... but was looking forward to a more reliable GPS system - but the input/search method on this thing is just as annoying as I remember a decade ago...

Besides the screen and GPS - is there any other considerations to the 200+ dollar price tag increase?  I might be making it up in my mind, but the audio sounded cleaner on the 8400... but I can't find any information that says the 8400 has some sort of better processing of audio or such...?

Is the Aux input on the 4400 also an AV aux input, or just stereo?  The 8400 comes with a TRRS extension cable which led me to finding out it supports a second camera - but does the AUX port on the 4400 also support video too for a second camera input?

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).

i bought the 8200(this was before the 8400) for the nav.   i think the nav is designed for europe, not that great here in the us.  however, i like the display better than waze or google maps.  and it is way behind on any new developments in our area, which there is a lot.  our other car has the 4400, and the difference in touch screens is very noticeable.  so, apart from the display, the pioneer nav is the worst.  because it is so far behind, forget turn by turn, and even planning routes is a joke.  i can run waze in the background, so probably apple maps will, also.  i dont think the display is worth the extra.  the 8200 has input for another camera, i would think this would also be available on the 8400