FYI to all Seems like TP-Link is finally releasing (under the Tapo family of products) a 4-way compatible option. Seems to be a 3 way dimmer kit (S515D KIT), and another non-dimmer switch that may be compatible (S515). Have not yet purchased as don't seem to be shipping yet but if anyone has any luck getting these up and running, would be good to know!

https://community.tp-link.com/en/smart-home/forum/topic/720504

  @ScottMan 

In you video, you had mentioned that if you install the smartswitch at the load side, the smart switch may lose power for a split second if one of the dumb sritched was flipped to slow.  My setup is where the line comes in, there is not a neutral wire there and i cant pull one.  However at the load side, there is a neutral wire available along with another line.  What would happen if i used another line wire to power the smart switch at the load side? Would this work and would this be an electrical hazard?  

  @board4mic Yesterday I connected the switches as indicated in the diagram, replacing the 3-way switches TP, with a standard 4-way switch, and the lights didn't work. Do you think configuring them in the app will work, or what else can you recommend?

  @ScottMan 

ScottMan wrote

@JHTOR as fate woud have it, I was just installing another set of 3-ways today and had TP Link's wiring diagram pulled up on my computer - so I modified it for this configuration.  The main thing you have to be sure to get right is being consistent on your traveler #1 and #2 wires.  Traveler #2 (black below) carries the load and needs to be tied to the Traveler #2 screw on all three switches. On the "4-way," you should have two wires tied to each screw.  Hope this helps!

I tried every possible iteration of this - with the original 4 way switch, with adjusting the 4 way switch to match the 'tied together' legs in the middle as well as adding another 3 way in the middle.

HS210 kit and a half - no luck.

At best I got the 'outside' two switches to work but the center switch is effectively bypassed. and I'd never get true 'state' reliably - it was always 2 button presses to cycle power remotely.

I'm giving up and switching to another eco system that supports 4 ways