I think everyone who is trying to hard-wire in a 4-way Kasa switch is missing the beauty of having "smart" switches.  I just created a 4-way using three Kasa 210 switches.  Here's how I did it.

In my upstairs hallway, I have a 3-way circuit with two of the 210 switches installed as designed / as the installation instructions tell you to. 

I also had a switch at the bottom of the stairs to turn on the upstairs hallway lights.  So when we're getting ready to go to bed and turning off lights on the first floor, we can turn on the upstairs hallway lights from the bottom of the stairs before we walk up the stairs.

With smart switches like these Kasa switches, and with the easy-to-use Kasa app, I don't need traveler wires.  I installed the third 210 switch at the bottom of the stairs and just connected it to power (black line), neutral (white), and ground (green/copper).  Then, I went into the Kasa app and set up smart actions so that when I turn the switch on at the bottom of the stairs, it turns on the upstairs hallway lights, and when I turn off the switch at the bottom of the stairs, it turns off the upstairs hallway lights.

  @Rusalka68 - see https://community.tp-link.com/us/smart-home/forum/topic/161800?replyId=1160712

I was inspired by @PatchyFog's awesome video and set up a similar test bed to try some ideas on the 4-way with dimmer configuration. I was surprised to find a relatively easy solution. The KS230 kits come with one master and one satellite device. It's clear that the traveler 2 wire is actually powering the load, as they are wired together. So traveler 1 must be how the switches communicate with each other. Since that communication has to be bi-directional, I assumed the internal electronics on traveler 1 are likely pretty robust and would be able to handle a third switch that could set or sense that line. So, I simply added a second satellite in series with the first and everything worked - all three switches could control the dimming and I could control the circuit using the app. Here's the configuration in order:

1. line

2. master - connections out on travelers 1 and 2, as well as neutral

3. satellite 1 - travelers 1 and 2 connected to switch and output, neutral pass through

4. satellite 2 - takes travelers 1 and 2 with load connected to traveler 2, neutral passes to load

5. load

Note this is essentially replacing the 4-way with a satellite where the input/output travelers are connected together. Unfortunately, I haven't found anywhere to just buy a satellite so you need to buy a second KS230 kit just to get the extra satellite. 4-way switches normally come at a high premium, so this doesn't seem too bad. This configuration could expand to multiple switches, but I don't know what the limitations are for the internal electronics.

I wish the KS230 would work in a 4-way.

Wasted 2 hours today trying to get it to work :(

  @NevHuntre Same here, we were trying to install dimmers in the 4 way setup, but could not get it to work (had an electrician here experimenting but they could not figure it out unfortunately). 

  @DavidDoria 

I've found that the 3 Way Dimmers don't work the same way (unfortunately).  I did replace a 4 way circuit in my house with a 3 way Dimmer kit (the KS 230), but "killing" the 4 way switch.  I bypassed it like you mentioned, which worked fine.  I think the dimming capability requires and actual dimmer switch at every location (maybe just so you can dim from either place).  Using the combo 3 way smart switch with "dumb" 3 way and 4 way switches only works if they are all just "boolean" switchs (on or off).   At least that's been my experience.  

  @septer012 

I was able to "smartify" my 5 way switch by simply replacing the "line" end of the circuit with a Kasa 3 way and that was it. The "other end" of the circuit didn't have a neutral wire in the box, so I had to leave the dumb 3 way switch on that end and the dumb 4 ways in the middle.  The lights come on and off with any switch.  And I can tell Alexa to turn the lights on and/or off.   I don't know if I can make it work with a 3 way dimmer, but I didn't need to dim these lights anyway.   It was a very simple and elegant solution.  However, it does mean that there are still "old style" switches mixed with smart switches which may bother some folks.  Just not me.  

In fact, if you aren't setting up a 3 way dimmer (which I have in several other locations), you don't need to swap out both switches.  You can leave one "dumb" and the smart switch will control the light from a wifi perspective.  Or you can simply flip the switch on either end to "manually" turn the light on/off.   This can save money by using only half the number of smart switches than you think you might need.  

  @rv75  Finally figured out my wiring issue.  I had correctly labeled the common wire but the HS210 initially didn't work well.  I guess it was because of the fact that I needed 2.  I swtich the wiring around and it made it partially work.  Dissecting the diagram is what really helped me.  The 2 wire cable was the common and I had moved it.  Now I have the travellers coming from the 3 wire cable and all seems to be good

  @PatchyFog Sorry to bug you again... I rewired my initial HS210 (hot black wire on common and red/black wires on traveller.)  I'm still having an issue.  Could the switch be defective?  When I use my second HS210 and the 4 way switch, everything works perfectly.  The minute I try to use the first HS210 to shut off/on the light after using any of the other switches, the lights go off and don't come back unless I start playing around with the other 2 switches and the app.  Something is not working right and I can't figure it out.

Just thought of something.. The only difference with the first HS210 I installed is that the "dumb" switch was not using the white neutral wires.  The HS210 needed them so I am using neutral now on that location.  Nothing else is different.