I have an issue with my 4 way setup...  I tried to replace the last 3-way switch in the chain, but it did not have the neutral wire bundle in the gang box.  I then went to the first 3-way switch location, and connected the neutral to the bundle of whites, the black 'hot' to the load/line screw, and the two travelers to the two traveler screws.   I flipped the breaker back on, but it did not work, so I flipped the travelers.   At this point, the light came on, but the Kasa switch could not turn it on or off, only the 4-way and the old 3-way on the other end.   I was going to install two Kasa 3-ways, but the issue is that the other end has no neutral to tie into, so not sure that I can connect it without that neutral.  
 

I don't have an extra 210 right now anyway, as I used the one from my two pack for my foyer, so I am debating on whether to get another one to try again, but first need to figure out how to deal with the missing neutrals at the last 3-way location.   Not sure why the Kasa could not control the power when it was added to the path, but power was working and controllable from the other switches.

Yes, Chicago_SH, they absolutely do detect current.

This thread inspired me to make a deep-dive video about all of these questions: https://youtu.be/kMT7KUVTELo

@PatchyFog Wow, what a great video!!   Sure, it's a tad long (I normally don't watch any tutorial videos that go more than 15 minutes or so, on the high end, or

I skip through them), but it was very well done, and I watched from start to finish.  I found it very informative and full of great example scenarios that are useful.  Unfortunately, the issue on the load side, in my case, is that there is no neutral wire found, only the two travelers and the hot wire (the hot is white).
 

When I connected the Kasa to the line side, the other two switches (3 and 4 way) worked, but the Kasa would not control the light.  The simplification shown in your video of the travelers and the alignment was greatly beneficial, and I'm thinking I must have missed something in my wiring when installing on the line side, but not sure.  I have to pull it apart and check again, but I'd prefer to install on the load side if it was an option for me.  I just need to figure out how to make it work with no neutral cable in the line side, but not hopeful.   
 

I did try all 3 wires from the old Load side in all 3 positions on the Kasa -- white to white, black to hot, and red to both traveler spots separately; white to white, red to hot, black to both traveler spots separately; I tried not connecting anything to hot, connected both black and red to travelers, and flip flopped them; and I tried the white to the hot screw, capped the white from the Kasa, and ran black and red to the two travelers.  This last way got me the closest, as it allowed the non-smart switches to work, but when they were both off, the Kasa just continuously clicked (it clicked continuously in all other configurations, too, but the other switches never worked, nor did the Kasa).  
 

I checked the 4 way, and it is only passing the red and black travelers, as expected, so not sure why the white is hot in the final spot...  This is the wire from the wire casing, and I cut it back to confirm that there was nothing else present, so this isn't a jumper that's white (while not ideal, it is not uncommon, at least in my house).  
 

I'm a tad frustrated, as that is the best spot to put this Kasa switch, since the line side has 4 switches in the gang box, and requires me to replace all of them to make this work since they are all currently toggle.  I am fine with the single 3 way switch in the corner of the bedroom being decora while the rest are toggles in the room, but they don't make a 3 toggle/one decora plate in the style that matches my other wall plates, so it's all or nothing in that spot.  

Thanks again for the excellent video!

UPDATE: I tried again on the line side, and I know everything was wired right this time, but I think I know why I'm having issues, though don't know how to solve it.  There are two Romex wire sets that feed these first two switches in the 4-gang box, switch 1 being the light switch that I want to replace with the Kasa.  Initially I thought the one wire was the incoming from the breaker (it is) and the other one was the outgoing for the travelers.  What I forgot, however, is that the second switch, which is also fed from the same 2 Romex sets, is controlling the ceiling fan on which the light fixture is mounted, so I think that the neutral is being used a bit differently for this setup, which may be why it's hot in the final 3 way switch in the series.  I need to confirm, but my memory is that there is a black jumper on the bottom of the single pole ceiling fan switch that is tied into the bundle of neutral wires.  Regardless, I was able to get it to work with the 2 normal toggles, but the Kasa could never control the light, and when the other switches were off, the Kasa would click repeatedly.  It may just be a lost cause without an electrician, as I'm getting a bit over my head. 3 way switches are easy, and 4 ways aren't normally too bad, but this one has me perplexed.  I've attempted the setup in both spots about 10 times now, and I'm just not getting anywhere.  All of my other Kasa 3 ways have gone in fine, but this is the first 4 way spot that I had to deal with, and it's beating me up pretty badly at the moment. At this point I'm calling it a loss, but still looking for a solution.  

@bjchristine I also had this exact same problem, I thought I had installed my Kasa switches correctly on both ends, but along the way I had unconnected the 4-way switch and then put the original 4-way switch back in.   So afterwards even though I could get both Kasa 3 way switches to work initially, any time I flipped the 4-way switch it would turn off one of the 3-way switches and then Kasa would no longer control anything.    

I found in my case the issue was that that may standard 4-way switch was actually wired incorrectly.   The way I had it wired, the 4-way switch was only giving power if it was flipped one direction.   So when flipped the wrong way, it didn't matter what was changed on either of the other switches, the circuit was broken and nothing could fix that circuit until I re-wired that 4-way switch.     

The important thing to know on the wiring for a 4-way switch is that there should be 2 black screws on the standard 4-way switch, both of those black screws should be connected to the 3-way switch.   The black screws do not mean connect both black wires to the black screws (that's were I went wrong).   You should have a black wire and a red wire both connected to the black screws.    My 4-way switch does not match the one in the diagrams shared in this thread, mine had the black screws both on one side of the switch.   

Once I had my 4-way switch wired correctly, then my 3-way switches started working wonderfully.   Regardless of which switch was manually changed, Kasa could always control the lights.     

And once I fully understood how all of the wiring works in these 3-way and 4-way configurations, I was able to determine which 3-way switch was the incoming line from the house and which one was the load going to the light.   I was able to remove the Kasa switch on the Load end and now all of my lights that utilize 3-way or 4-way switches only use one Kasa 3-way switch for each circuit.   The other light switches in each circuit can just be standard cheaper 3-way or 4-way switches. 

@PatchyFog  TP-Link should add your video to their support site. That is super helpful in explaining pretty much everything about these 3 way switches.

@spatical Thanks. You have excellent taste! I'd be happy if they'd simply comment on whether the "dead" traveler voltage is artifactual or intentional.

Bought a 6 year old high end custom home with a 4-way switch that I wanted to convert to a smart switch. Had a handyman with an electrical background successfully install several Kasa smart switches in other parts of the house single switches and 3-ways. Tried repeatedly to get the 4-way to work with the Kasa 3-way switch pair, leaving one dumb switch. But could never get it to work. 

Finally had a real electrician come out with experience with smart switches. He found out that my 4-way had been originally wired in a dead end configuration, and therefore impossible to get a smart switch to work without rewiring the lights and switches in the walls. 

So that's that for me. FYI, if you're having problems figuring out why you can't get your 4-way to work...

@spatical brilliant.  Thank you. 

OK I am looking at an original 4 way setup with one switch having the expected 4 way wiring, one has the expected 3 way wiring, but one has what looks to be a 3 way switch but wired in a different way. There is the expected black, black, red but with an additional black on the line side. It appears to me that the 4 way switch is at the start of the chain, the 3 way with the additional black wire is the middle, and the last is the standard 3 way. The additional black is power out to the last 3 way? Is that possible? Anyone seen this setup before? 

I have installed two kasa on the 3 ways, combining the blacks as they were originally. The lights work but depending on the on/off state of each switch, the other switches don't always work. 

Any suggestions?