Update on the HE-4, basically confirming my previous opinion:
Surprisingly comfortable headphone due to soft headband (I am serious), low weight (in comparison to other Hifimans) and absolutely tolerable clamping force (much better than Audeze, not as good as with HD800). Subjectively a well-constructed headphone - I am not worried about manipulating with them or carrying from place to place. Also quite compact. Definitely happy in terms of overall build quality.
Sound quality is absolutely superb for the price... Stunning macrodetail, tight+deep analytical bass, mild U-shaped FR (not offensive by any means - mids are still there... just not lush or warm, rather neutral), quality soundstaging (that width and separation!). They are fast and clear sounding... HE-500 simply cannot keep up with them in terms of technicalities, though I would call HE-500's FR a bit nearer to neutrality (both HE-500 and HE-4 not completely there).
Downsides of HE-4? None if you consider their price and other products in the same range. If you compare with significantly more expensive headphones, then I can hear some lack of microdetail and higher distortion somewhere in lower treble, resulting in a slightly less clean sound... I agree with purrin that general timbre is slightly off with these headphones - but it's not as bad as it can look like. HE-4 are just the analytical Hifiman headphones - more technical and less natural-sounding than something like HE-500. As the headphone is slightly brighter than neutral + provide a good portion of air, background is not as black as with something like Audeze headphones.
For me, they belong among the best headphones I've heard up to 1000USD, rivalling LCD-2 and HE-500 for sure (slightly surpassing them IMHO). Simply a hidden gem - IMHO.
IMHO.