Four or five years of community requests and still no product. This is starting to make me think TP Link doesn't want to support the demand of the community and I should consider a different brand. I'd be purchasing a handful today if you had a product.

If you don't want to make a 4 way product, how about an affordable middle switch to control the master? In other words, we'll wire as a 3 way with a middle switch that controls the function of the first including dimmer. This isn't a complicated solution but I do not believe smart actions allows the control of dimming. It essentially just needs another satellite that can communicate over Wifi.

  @MrShizzle 

This works for me with HS210 and one 4-way switch. Only difference with the schematics that everyone had posted is that the neutral wire of the hs210 of each ends is connected to the neutral line and to the light. 

If Kasa made an "official" 4-way wifi switch I would buy them for my house.

  @Chicago_SH you only need to put one switch on the power supply end, Right Side on your diagram. If you buy the two unit kit, you have two switches to use in different locations. The 3-Way Dimmers, now that a whole different story. Just got done doing this exact thing. And agree on the kudos for the great gear!

  @Carl if i can vote for a 4 way switch option, id like to. I need several 

If I'm willing to eliminate one of the switches in my 4-way (and just put a blank panel there), will the kasa 3-way dimmer kit work? 

  @Beaugas were you able to get this situation remedied? I have the exact same issue. 

  @JM1010101 in my experience if the line and load switches (first and last) are installed correctly it can work.  However, I did get a "switch is incorrectly wired" note and had to turn breaker off then back on and it worked.  The lights sometimes are delayed in responding but the two pairs that I've installed work with the app just fine.  

  @board4mic 

Is there any way to get dimming to work in this scenario, even if the app cant control it?