For the price the build quality and sound reproduction is unmatched. Listening through a Schiit Jotunheim is a pleasure that has not lost its luster.

The best

I'm coming from legacy 3's, the T2, T2 plus, T4, bl03, ER2XR&SE, ER3XR, ER4XR and a bunch more. I find that the tuning is exactly what I want in an IEM, but the soundstage is somehow lacking compared to my legacy 3. Zeos mentioned this in his review, and I didn't want to believe it, but it's pretty noticeable. I'm really not as disappointed as Z was though. It's a fantastic IEM.

Bass response is low and soundstaging is almost non-existent even after 200 hours of burn in.

Impressive detail with a light and clean sound. Nice stage and decent imaging. Bass is tight with a nice rumble, but it doesn’t kick hard enough for my tastes.

Extension both ways with great detail and very good timbre. 5 stars for value and a complete reproduction of sound! If you enjoy separation and layering these are sure to please. I feel like any musician could use these on stage and get effortless clarity and would hear each element of the mix distinctly. If you like balanced armature sets but always feel better with the 'Bass Boost's on, these deliver the low end - and it's fast and textured almost as if the DD were a large BA.
These are an upgrade from my ikko OH1 and Tin P1. Bass is not quite as lush and full as ikko OH1 but the treble detail and extension is much better on the Legacy 4. I love the clarity of my P1s but vocals don't always sound natural. To me, the Legacy 4 delivers more pleasing vocals - even if they are just a step back in the mix. I was hoping for a bit wider soundstage but it's still there. I would recommend silicone tips. Experiment with different sizes and nozzle types.
As for the dip switches... leave them down/down (default). For me they seemed to decrease impedance and made them feel overly sensitive to the amplification I was using. I have used a slight midbass (125-300hz) boost with EQ to warm up the vocals and lower mids. Like most BA sets, poor recordings can get a bit shouty in the 3k-6k region but the treble is quite well controlled by the filters and configuration they are using.
For $200 IEMs these are an excellent value! Not too warm... Not too dry... Plenty of detail... Yep!

DD Qaulity = Very good
BA Qaulity = Very poor, Too harsh at high frequency. Texture is unnatural.
Too much hyped.

Just a quick summary of the L3 / L4/ L5 for those on the fence on which iem to get:
L3 - Mid bass slam with solid mids and non-threatening highs. Fun. Affordable.
L4 - Modest but tight sub bass focus with clean mids and crispy highs. Narrower sound stage than L5. More resolving than L3.
L5 - Mid to sub bass focus with warm mids and smooth highs. Wider sound stage than L4, with silkier highs.

Buy the L3 if on a tight budget. Buy the L4 if you prefer sub bass and crispy highs. Buy the L5 if you prefer an overall warmer, smoother tone. I like all three, although for my preference I'd prefer to have the Bass of the L4, with the Mids and Highs of the L5.

After spending a week with the Legacy 4, I will share my thoughts after 50 hours of running in. I did note these sounded better the more I listened to them.

PACKAGING is superb for an IEM of this price. Although a bit over the top, it is a welcomed addition. Unlike the faux leather cases of old, the new case has plenty of room for your IEMs. The older cases were extremely snug getting the IEM shells in.

FIT is very form fitting. The pseudo custom shells are very comfortable and are on the smaller side. It will suit a lot of ears.
When it comes to tips, I found the softer Spinfit 145 to be the best fitting. When paired with my go to Final Audio E tips, I either had to go for a shallow fit or drop down a size for deeper insertion. Final tips were tricky to get a consistent seal. Shallow insertion, you lose the seal fairly quickly and have to constantly readjust. Too deep insertion can cause a suction effect, drowning out the sound. There was more effort in this balancing act with Finals' tips, so Spinfits alleviates the trouble.

TONALITY is well balanced. I did most of my listening with the stock cable and noted sub bass has more emphasis over mid bass with great speed and texture. The lower mids are more relaxed while the upper mids are more energetic. This liveliness carries through to the treble.
After a few days, I paired the L4s with the upgraded EST cable and my jaw dropped!
There was some added warmth. The low end became a bit more impactful and the midrange became fuller. Keep in mind all my listening is done on the default switch settings.

TECHNICALITIES is great for the price. Do keep in mind these thoughts are based on using the IEM with the EST cable. Detail retrieval is great and I'm able to hear the nuances in my test tracks.
Timbre sounds very natural when using the Spinfits, everything from bass quitars to cymbals. I did notice slight smearing of percussion instruments with some other tips however, YMMV.
Coherancy is fairly decent. Rarely does one instrument jump out at me, but don't expect the same level of coherancy as a single dynamic or higher driver counts (generally speaking).
Finally sounstage. I would describe the soundstage as above average in width. The L4s also manages to get a good sense of height also. Depth and layering originally I felt was somewhat lacking, but seemed to have opened up with more burn in.

COMPARISONS. The closest IEM I have in this price category is the NF Audio NM2+.
The NM2+ excels in coherancy and sounstage. I rarely get an out of head experience with IEMs including ones where other reviewers have commented on how "holographic" they can be. The NM2+ can at times push the boundaries of my headspace and the L4 is not far off that. Due to the more transparent nature of the NM2+, you really get the sense of space. It is also this transparent nature that limits the NM2+ from certain genres/ tracks and can be unforgiving at times. I prefer the Legacy 4 for their versatility and ability to handle anything I throw at it.

For sub $200, you're getting an excellent IEM that gets everything so right. I would like to think of this as a budget end game until it's time to buy the Clairvoyance/ Monarch.
My closing thoughts is that I like the Legacy 4 a lot! And even more so with the EST upgrade cable. For me personally, there isn't much if anything I can complain about.

I look forward to see what ThieAudio and Linsoul plans for the future!

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.