I love my NEX 4000. It's superior in every way to my appradio 2.
Things you get over your 3 on the 4
Bluetooth folder browsing, including iphone
iTunes Radio support for the built in media player.
Carplay
More audio sources.
Higher voltage preouts
HD Radio (I use this more than I would have imagined)
Actual media support for things like SD cards, and flash drives.
A second USB port to do with whatever you like
An aux jack as well, and much more random crap.
Mind you 95% of what my NEX can do I don't touch, but if any of the above things are worth spending the money for you, do it. I for one just wanted carplay.