So I should preface this with the comment that the ideal signature in a closed iem for me would be what the i4s have, with perhaps a little bit more warmth. This also surely doesn't help, but now having the stable of the i4, Solaris, phantom, CL2, and Atlas, I truly have zero desire for any additional acquisitions (famous last words, but I have a feeling this will stick for quite a while). They each capture key pieces of the i4 signature, but none of them are perfect analogues, and none of them really get to the same level of open soundstage that the i4s have because, well, they're not open.
The CL2s, once burned in, have a lot of the speed, detail, transparency, cohesiveness, and comfort I love about the the i4s and which are characteristic of planar drivers. They also capture one of the i4 x-factors that are really rare in iems, which is that sense of effortlessness that the i4s have that makes it seem like sound is just instantly, effortlessly conjured out of thin air, rather than coming from a driver less than an inch from your ear. This adds to their transparency and their seeming to disappear. Their bass is similarly authoritative and deeeep when in the recording and their treble has a similar, somewhat laid back, but hyper-detailed sweetness of the i4. The CL2s, however, have much more energetic upper mids, which, especially before they are fully burnt in, can slip into harshness. They also sound thicker than the i4. Though they have a decent soundstage, it is nowhere near as big or open as the i4. A warmer copper cable definitely helps the energetic upper mids. Getting an 8-wire copper cable also helps with the openness (doesn't need to be an expensive one--$30-40 **** or kinboofi 8 or 16 wire copper cable does it).
The Solaris may come closest to the i4s overall, but just a little bit warmer. They are definitely the most cohesive hybrid iem I've heard. They have a similar emphasis on subbass as the i4 and have similarly great extension, but decay is slower (not necessarily worse, just a different characteristically). Clarity, texture, layering, imaging, and space in between instruments is tip top, equaling the i4s. It also has some of the largest soundstage I've heard from a closed iem and all these things come together to give that same holographic quality the i4 has. Mids on the solaris are similarly silky smooth (but detailed) as the i4, although they are, to me, a little more forward. Where they differ is in the treble, with the solaris treble being more sparkly and energetic without ever slipping into harshness. They also don't have that effortlessness that the i4s and CL2 have. They are pretty new (got them about two weeks ago), and I think the bass may still be changing (see below on my thoughts on atlas, which has a similar dynamic driver and required a lot of time to settle into their final sound).
The phantom is much warmer than the i4s across the spectrum and have a much thicker, more intimate sound with rounder notes. They do however, have excellent imaging and transparency and also have the holographic, 3-dimensional qualities of the i4. With the warmth, there is more of an emphasis down low, with great texture and speed characteristic of BA bass, but with a more organic decay and resonance than you typically get with most BAs. It’s not dynamic driver (or planar) bass, but it is fuller than most BA iems give you. Even with the warm presentation, they are fantastically linear across the spectrum (also fairly i4 like). the Phantoms can, and do sound more lifelike in their timbre and with many instruments, particularly cello, than any other iems I have heard, including the i4s. It is a colored, but very natural, organic sound.
The Atlas is a visceral, fun, big sounding iem. It, like the CL2, and to a somewhat lesser extent the solaris, also requires a lot of time for their sound to settle (100 hrs plus). They produce a real physical wall of sound Once burned in, they have a lovely, very cohesive, and, when in the music, very bassy signature. Like the phantom, I find it a very natural tuning with very well rounded notes, albeit weighted down low like a medicine ball. The bass doesn’t not spill into the other sections, which is quite a trick. Mids are more neutrally placed and farther back than on Solaris, phantom, or cl2, but (once burned in) not recessed. Highs has a nice amount of air, have what i would call a sheen but not quite sparkle, but are not fatiguing at all for me. The Atlas is very forgiving and makes even poorly recorded/mastered music sound great. It is well detailed, but not hyper detailed. Because of its physicality, it is not a relaxing iem, but it is an addictive signature foe when you want that extra grunt down low.
Hope that’s helpful!