I purchased these with the upgraded EST cable (more on that in a second). And I also own the Monarch and Moondrop Starfield. Not tons of experience with high end IEMs other than that I've heard Audeze LCD-4i and Sony IER-Z1R for brief periods in the past. But I have listened to a lot of headphones and speakers in my day.
These sound very good tonally. They are a touch forward, but I don't consider them bright, some might. I find they have a great balance and body to them. The EST cable is a must for me though. I got them because I liked the ones on the Monarch so much I wanted a second set. They do actually sound better though. The stock cable if great. Super comfortable, soft. Heck, even drapes better than the EST. But it does sound more hollow, less focused, with a touch less sub bass. And the stock cable smells funny out of the box. The EST is really worth the upgrade when comparing to the Monarch.
So, how does this sound compared to the Monarch? Really, really fine indeed. I actually slightly prefer a couple things. But it obviously isn't quite as good in some areas. The Monarch has better micro detail, depth and slightly smoother high end, even though it's a little more detailed. But for tonal balance, some might like the L4 better. The L4 has a little, tiny bit of extra sub bass, the Monarch has a fair bit extra. But the mids are not as thin on the L4. The texture and quality of the bass and sub bass are also noticeably better on the L4. Yes, better. The drive in the L4, in my opinion is superior and Thieaudio/Linsoul should be working on a Clair/Monarch right now with an upgraded version of this driver in it. If you EQ the L4 up about 5db in the 30hz region, you pretty much have a Monarch with better quality bass, better mids, and not as good highs or technicalities (aside from bass texture). This is of course with the EST cable and the same SpinFit CP100 tips (medium for me) that come with the Monarch. The stock tips are actually really nice, but don't quite get the bass where I'd like it, and can be a bit much in the lower treble region, which is the only place this IEM struggles in my experience.
Now, I'm not being hard on either the Monarch or the L4. I think to be totally fair both are a fantastic package and some would take one over the other. I, however, would take a blend of the two for the Monarch's price. But for the L4's total package - oh and the packaging and accessories are miles better than the Monarch - I would buy the L4 instead. The ONLY caviet is that you give up some bass quantity for quality - a trade I'm happy to make. And you do give up a near perfect treble response for a slightly, slightly more forward and less refined one.
And the fit... This is going to very from person to person. The Monarch fits me like a custom IEM, but is large. The L4 is smaller in every dimension, and fits me extremely well, except for being a little deep due to the narrower body. So the tops touch my ears and the bottom and nozzle I don't insert fully. I insert them all the way, then pull back maybe 1mm. I find them just as comfortable as the Monarch, which is saying A LOT. But not as easy to dial in right away. They actually do seal and find a better fit faster with the stock tips, but as I mentioned above I like the SpinFit CP100 sound better on the L4. The fit really does put the Audeze LCD-4i and Sony IER-Z1R to shame. And the sound for exceeds just passable.
The only things I don't love about these is the lower treble, like I mentioned, and maybe that they are very sensitive and will pick up source noise. Not that I knock them for being super easy to drive and pair with modest amps/DACs. My Cayin N3Pro can make a few tiny little squeaks on Bluetooth LDAC mode or with the tubes if I toss my phone on top of it. But in normal use, it's quiet with these and my Dragonfly Cobalt is dead silent, so no issues there. But I can tiny noises here and there that other IEMs don't pick up.
These are good. You will like them. I would recommend the EST cable and trying some tips (I like the CP100s). But even bone stock, you could do far worse.
Now Linsoul/Thieaudio. Hear me on this. Put this driver, or a better one in the Monarch. It's an awesome, awesome IEM. But the lack of bass texture really is not quite at the level of even this L4. Perhaps the Clair is better. Maybe it's the balance making it seem that way. But I've EQ'd these, and the bass is just better quality on the L4.