If the Note 4 still works well don't ever change! It is the best Android phone ever built. Nothing else comes close besides Note 3
OK, so my 3 now runs a modded S7 ROM and runs a 2 week battery usage as a kitchen clock with no antenna active.
It filled with water in the headphone hole and would not give me sound without wired headphones plugged in. I cannot do wired headphones. One of my trial custom OSs ran really hot, drained the batttery in half an hour but at least dried it out.
In the mean time I got my 4 for £40 instead of £400 on a pricing fail.
The 4 is a pile of awesome, wrapped in excelent, sprinkled with magnificent and then marred slightly with a couple of rediculous but minor changes to basic functionality I will not go into as they may have annoyed only me.
I kind of messed about with root stuff too much on my 4 and caused some glitches that would require a start from scratch and I needed a phone and had also reverted my 3. Bring on the 8 to set up fully while using the 4 until the switch.
The 8 abosultely, and with no question is so much better (with the usual caveat that the changed all the alarm tones again so back up first and a few others)
Speed: 8 did 97 app updates to the 4s 23 in the same time during setup.
Screen: just so much nicer.
Edge: turn it off if you want.
Camera: Wow, some Amazeballs, a sprinkling of some more Wow and a great deal of being extremely impressed at the diofference, especially in low light. Never mind the fancy doodads.
Aspect ratio. Was odd at first(it's long) but feels just right now.
UI: Not entirely sure as I use Next Launcher butr have also started using the edge thingy and find it nice for most common contacts and apps.
Pen: not much new but feels a little nicer in the hand.
Nougat: Realy superior OS, app and power management. It's a smaller battery but can get more out of it.
HW Buttons: Was over them in a day. Makes much more sense having them software.
Smart Lock: Bypass lock in trusted locations/bluetooth connextions etc.
Iris unlock. pretty damned reliabale and more secure than face unlock
Serious or blatantly annoying Note failure since the start:
Alarms on the 8 now sort by alarm time rather than last edited time
After the Not 2 you could not change the tone played at timer completion. You can on the 8
Exploits:
There are a shed load of android exploits that have never been patched on the note 4. This is only a problem if you download apps from dodgy places or on the rare occasions where stuff gets through the store..
The store is cleaner now but later versions of Android are safer against these.
My only complaint about the Note 8 over my 4 is that the play duration of alarm tones has gone from 1 to 5 minutes and there is no way to change it.
Oh and the tone for the timer now plays over the notification channel and not the alarm channel so cannot be played during quiet hours despite effectively being an alarm.