Took me a while to figure out how to install this in a 4 way configuration.  I am just posting my conclusions and welcome some feedback from folks who

have more info on how these kits actually work.  My theory of operation below is just conjecture, although fits the observed facts. 

 am trying to explain why the kit behaves the way it does, instead of just providing a set of tips "do this and that, and do not ask why".

1) The kit consistes of a smart master, and a dumb slave switch. So folks who naively expect to get 2 smart dimmers out of the kit
will be disappointed. 

2) The master gets line, neutral and 2 traveler wires. 

3) the slave get the 2 travelers, and load. 

4) Both switches must be connected, or the self-check will flag a wiring error ( flashing red status)

5) It does matter which traveler is connected to which on the master and slave.

6) It seems that a dumb on-off switch in the middle can work ( sort of) , although it has to be in one particular position, effectively

acting as just "wire" between the tp-link slave and master.

7) From a wifi/kasa point of view, only one switch is visible and controlled, the master switch.

8) Wiring fault status is "sticky". I.e. if a wiring change is made, the reset button must be pressed again to re-run the wiring check

Only then will the status reflect the current wiring reality. 
 

A few observations:

The neutral wire is needed to make the status led on the switches work, since these need to be able to light up 
even when the load is off.   The master switch somehow powers the status light  of the slave switch.

I suspect that the master and the slave communicate, to make the dimmer function work  on both of the master and the slave. 
This communication is necessary when the self-check function runs, and failing that a mis-wired status occurs.
 

I am thinking that the middle switch in a 4 way setup in one particular position just criss-crosses the travelers, and the master and slave switches
expect these to be connected  #1 to #1 and #2 to #2, for the master-clave communication to work.  

I wonder if there is any words of wisdom on how to wire up a 4 way setup? Is there a 4 way kit?  My solution works well anough, although feels a bit hack-ish, 

and it is effectively regressed to a 3 way setup, ( the  intervening 3rd switch has to be kept  in one fixed position).