If you go over to the i4 thread there are a number of us who have both the i4 and the isine20 and have given our impressions around the differenve between the two. Simply put, the i4 is on a completely different level from the isines 20. It is a major and immediately noticeable step up in terms of sound quality in pretty much all aspects--resolution, transparency, clarity, soundstage, bass extension, bass impact, treble extension, soundstage, midrange, prat, etc. I'm not going to put a number on the percentage improvement because that's so subjective as to be meaningless, but suffice to say there is a major, immediately noticeable improvement across the board with the i4. The isines and i4 are clearly in the same family, as in the physical design is very similar and both are semi-open, non-isolating iems, but the i4 is simply in a different class, head and shoulders above the isines.
The i4 is more power hungry than the isines, so while it still sounds good out of an phone, you may not be able to get the volume level you want plugged directly into a phone. You will want to use an amp of some kind to get the most out of it. It doesn't need to be a super powerful amp--I have found the i4 sounds quite great out of my cozoy REI, which is a dac/amp with no battery that uses power from your phone/dap/computer (the volume does need to be almost all the way up). It sounds spectacular out of a mojo. i4 also scales extremely well with higher quality sources and amps.
