I have a pair since end of summer.
The left diaphragm is perforated (tiny 1mm² hole), so when loud bass notes are played, there is a rattle noise. I have been using them for 3-4 months, I wonder how long they will last with a torn diaphragm. Sometimes I push them around 85 dB. I paid little money for them because of that issue (+ channel imbalance) so I really dont mind.
I usually don't EQ headphones. I have done so during many years, often seeking good tonalities without weird dips or peaks in the FR.
The HE-4 is honestly a very bright headphone in stock form, and the upper-mids are missing some energy. The mid and upper-treble are very nasty, the whole 7-13 khz region is very elevated and gets on my nerves very quickly.
I have owned some classic Hifimans in the past, such as the HE-6, HE-500 and HE-5LE. Always liked the punch and speed of these cans. The HE-500 had by far the best tonality in stock form, but it was the most compressed sounding of the bunch, with unrefined treble and slightly (but still impactful) loose bass. The HE-4 definitely has a solid combination of fast transients with great attack and dynamics. One of the best "blackground" and clarity I have ever heard in a headphone. The HE-4 reminds me of a baby 4-screw HE-6 with less bass and mids body.
Considering the issues my pair has, and the annoying stock FR, I decided to keep them and play a bit with EQ. They respond nicely to EQ, especially in the bass and treble. They aren't the cleanest headphone, with some distorsion in the upper-mids & treble, and the zippy treble is still there after EQ, but the results are so satisfying that I decided to EQ them by ear with my HD580 next to them and get as close as I could. Of course, it won't ever have the nice timbre of the Sennheiser, and the sweet midrange, but I got good results. The bass is very taut and can slam relatively nicely after a boost.
With proper amplification (Bryston BHA-1, but they would actually also benefit from a good speaker amp that rolls off a bit in the upper-treble), and some additional modding (felt/dampenings), they're really a treat. I use mine with stock velour pads (the old Hifiman velour pads with fragile tabs). These pads have a larger opening than modern Hifiman velour pads and are softer ; more comfort and slightly better stage. They do sound a bit brighter though, so an EQ is definitely mandatory for my personal use.
Pic of "modern" velour pads on left, old velour pads on right :