The AP4 has more bass than AP2, but I'm not really sure it's "better" sound if I'm being honest
It is "thumpier" sound, which I do realize some really prefer
(this is basically the whole value prop w/ Beats products)
I took AP4 on a long hike yesterday and honestly found the music a bit fatiguing eventually, somewhat due to all the bass
Just adding a data point
When comparing with AP2, one has to realize that the design and how much is getting right into your ear (vs escaping to the sides) impacts this. I have some tips that direct the AP2 opening/sound much more into your ear canal and it's pretty shocking how good AP2s actually do sound, when the sounds gets into your ear instead of escaping.
I really believe this accounts for a majority of perceived sound differences for users