If you have four wires it sounds like you have a 4 way switch. When installing the HS210 you will need two traveler wires in order for the power to pass through to complete the circuit to turn on the light (Three way switch). So it would not work.

I just went through this exact issue earlier today. I found a helpful FAQ but it didn't connect all the dots, so see if this helps:

For a basic 3 way switch (only two switches) look on the old/dumb switch for a black screw or printed/embossed 'line' 'hot' or 'load' text. This is your 'black screw' wire for the TP LINK switch.

The other two wires on the switch (usually red and another black) are your 'traveller' connections for the switch. The ground is obvious as either unshielded copper or a green wire similar to your smart switch. That leaves the neutral to be the white wire, usually tucked at the very back of the box with a twist-cap on it linking to the other switch. Take off the twist-cap and add your smart switch white wire, then re-twist the cap.

You SHOULD be done at this point, other than testing and then putting it back in the wall.

Note - the hs210 switch came in a two-pack for me, the packaging suggests you need to use both on the same circuit, but you don't. Once you get one of the smart switches wired in correctly you can put the other in another 3-way switch for a different circuit. The app will ask you to configure both for the same light during setup, but after you have the first switch configured and it asks if you're ready to do the second one just force-quit the app and re-launch it while using the switch on a different circuit. Presto! I bought 3 kits and wired 6 different 3-way lights this way, works flawlessly.

If you do actually have a 4-way (3 or more switches on one light) - put it back in the wall and go to one of the other switches on that circuit and open that up and hopefully see a typical 3-way dumb-switch wiring job. That's the one you want to replace with the HS-210 as I described above (again, just went through this exact scenario today).

Here's the easiest way to view what is going on with a 3 (or more) way switch. the two at the ends are always basic 3-way wiring, the one (or more) in the middle are 4-way.

Thanks.  Was just going to ask about using the switches in the two pack kit together or using each with an existing 3way switch.  I figured you could do that and appreciate that you posted on how to make that happened.  That will save me some money.

Thanks KriegTiger for your post #3. This has helped me. I followed the TP Link instruction video for 3-way switches. I assumed because the Common Wire in the video was Red, then my common wire must be Red as well. When I was done, what I had was a Smart Switch that would Turn Off the Second Smart Switch; it was very humorous and frustrating. After following your suggestion, I was able to get them wired correctly and took pictures that will hopefully help someone else

The labels are blurred b/c they are labeled Incorrectly.

@KriegTiger Hi,

I have a question for you.  I have two HS210 switches.. I installed one of the smart switches first and did not install the second one..  So I left the dumb switch in the on position and the smart switch worked fine.. I want to add that I named the switch ceiling light.  The problem is now I have installed the second smart switch to control the ceiling light; however, I can control the light with both switches; however, one of the smart switches seem to be turned on and the other one seem tobe turned off.  I'm not sure if during the setup of both switches are paired together, because in the app it has the lastests (the last switch I installed) switch listed has offline.

So does

1: does both switches need to be paired together?

2: should both switches be named the same?

3: should I remove them from the app and start over with the setup, but just leave them installed in the wall?

Thanks.

@KriegTiger - I have a 4-way configuration, but none of the switches have a white neutral wire. If I connect the white to the bundle in the box, the physical switches all work fine, but the app and Alexa only work every other command - ie: i have ask Alexa twice to turn it off/on. Is there any way to correct this?

@derickMoonPilot 

So my wiring has a black as hot, red as traveller, and a white wire that is not connected to the twisted neutral wires, perhaps another traveller?