Pros: Great build quality, comfortable design, 3.5mm detachable cable
Cons: Congested sound quality, doesn't fold up, earpads not replaceable
Along with the summary below, I have posted a Youtube video review of the UE4000. If you like the video, check out my channel for more reviews :)
The UE4000, like all the new Logitech UE headphones, has some really nice build quality for the price. Solid feeling headband with metal parts, supple synthetic leather earpads. The detachable cable with iPhone remote is a great touch because it terminates in a 3.5mm jack and can be replaced with any off-the-shelf cable. The earcups pivot in all directions which makes getting a good fit easy. Unfortunately the ear pads are not replaceable and the UE 4000 does not fold up into a more compact shape.
The UE 4000 has a bassy, rich sound, but they have quite rolled off highs. They don't completely lack energy in vocals but overall the sound is quite congested and separation / layering really suffers. This kind of signature is good for a noisy commute (you don't have to turn up the volume as high to get a decent sound) but is just not a great balance otherwise. The UE 4000 is less balanced than the UE 6000 (which I liked).
These would make a good headphone for people on frequent noisy commutes, or a nice little package for someone for whom sound quality is not the top priority. They aren't terrible sounding, but the congested signature really isn't my thing.