Yesterday I received the ZS4 and this was almost 2 years since I received the ZS3. When I ordered the ZS4 there was no indication whether KZ would try to maintain the ZS3 sound signature for the ZS4 or change it completely. The shells of these two earphones are almost identical except that KZ added a lip on the nozzle of the ZS4. When I compared the sound of the ZS3 to that of the ZS4 what I heard was two very similarly tuned earphones. The big punchy bass was maintained, the detail in the mids and treble was also still evident and the treble seemed equally extended in the two. The ZS4 seemed to go deeper into the bass and the treble seemed to be slightly more forward. The ZS4 like the ZS3 was sibilance free. I'm sure others here with better ears than mine will find more difference between these earphones. To me KZ kept all that was good with the ZS3 and added some bass extension and elevated the treble a little bit. I like the ZS4 however I personally would have liked a little more emphasis in the treble as I think this would have added a little more difference between the ZS4 and ZS3.