I can repeat it again with pleasure.
The i4 is certainly one of the very best in-ear you can buy. Its competitors are Overear in the range 3-5TUSD. It has a sound that is addictive and hard to describe. This clarity, velvety, never garish or disturbing, a bass to kneel down. For me it's clearly a convenient replacement for a huge Overear.
But to develop its potential, it definitely needs a big amplifier. Not one of those mobile things, it's just not enough, the more power the better. The crowning glory is balanced on a tube amp! Then everyone will hear what's inside this little thing. Massive. For me, the headphone of the decade!
Which is just a pity if you use it mobile via cipher or mini-amp. It's just a waste of money. Then I clearly recommend the i3. You save a lot of money and the i3 is very close to the i4 in many ways. I would say that with the i3 you have 85-90% of the i4. The i3 sounds absolutely perfect with cipher! That's why I bought the i3, just for iPhone and so on. The i3 has a much better efficiency and the cipher is excellent, a dream sound for a mobile combination. i3 sounds also very good on the amp, on my iCan it comes close to the i4. But the i4 is unique. It does not achieve this and to pay more than double for the last 10%, yes, it was worth it to me. But if you would only listen to the i3, it is perfectly sufficient and everyone should be happy with it.
What you can forget about is the BT solution. Yes it works, but it is clearly a stopgap solution and you lose a lot of the sound. Then it is nothing more than a slightly better AirPods Pro. A pity for the money. If BT, I go directly to the AirPods Pro. But with cable via cipher, the i3 a dream, on the cipher the i3 beats the i4 quite clearly! If you just want to listen to your headphones on the go, you can save the investment in the expensive i4 and use the much cheaper i3. But if you want absolute totl and absolute high end at home, you buy the i4, but please use a decent amp. Otherwise you give away your potential and are clearly better off with the i3. The i4 is for connoisseurs to listen once! Especially with very good recordings via Qubuz and Audirvana.
I prefer listening to operas and classical music with the i4. There is hardly anything better.
There is much discussion about EQ. Yes of course, it is a very personal thing. I like the i4 best of all absolutely neutral, without EQ, just via iCan in tube mode. I miss nothing. Yesterday I installed the plugin from Audeze. I was extremely disappointed. It destroyed the i4 right away. It sounded awful. I didn't even like it on the i3 and the LCD-X. With the i3 and LCD-X it sounds much better to make a very minimal correction via ADI-2. Much better. I would never spend the 200 USD for the plugin, which is an absolute disappointment for me.
But recommend everyone once, take your time and test the i4 on a very strong tub amp in balanced mode. You will kneel down, what a marvel of technology this is!
And as mentioned before, if you want to use it mobile via cipher or small amp, save the expensive investment and take the i3, it's great!
To the iSine. No, don't buy it anymore. It is sold out almost everywhere. But the i3 is a massive improvement on the iSine20. I still have it, I will keep it, you won't get anything when you resell it. But it is more a fun listener. And yes, it needs massive EQ to make it enjoyable. But in between I will use it again with pleasure, so that you know what you have on your i4!