Hello, 

This post is about a very hacky way of replacing a 4-way (4 wire) switch with an HS210. This is very not recommended so I'm posting it only for educational purposes - Don't do it !

Say you have a light circuit at home that is controlled from 3 separate light switches. This means that the circuit is composed of 2x 3-way light switches + 1x 4-way light switch. Now, you can easily directly replace the 2x 3-way switches with HS210 switches and have full control of the circuit via the app.

See here for details: https://community.tp-link.com/us/home/forum/topic/161800

Now, you're left with a 4-way switch that the regular non-smart type and looks out of place. It works, it just looks different. 
It is possible to replace this switch with a smart switch by using:

• An HS210 switch
• A 120V AC DPDT relay - I used this:  https://www.amazon.com/itm/dp/B00REFBH88
• 2x1N4007 diodes - https://www.amazon.com/itm/dp/B07Q6J9TNW
• Hot glue gun.

Basically, the relay will be the actual 4-way switch and the HS210 will control the relay. There is no need to connect the HS210 to your account / app you'll continue to controlling the circuit via the other 2 HS210's on the circuit.

Pro's:

- I can get all my light switches to look the same 

- The entire setup fits in an electrical box

Cons:

- This probably violates electrical code

- The relay tends to get a bit hot (60C)

- The indicator light on the 4-way switch won't actually show status - it will have a random status and will not update when you turn lights on/off via the other 2 TP210's

- Extreme hack

The circuit Looks Like this: https://i.imgur.com/nTjJu8s.png

Pic of soldered mess: https://i.imgur.com/YIHSmqf.jpg

Of course this was glued so that nothing was exposed and then installed.